Forge

Forge is a gameplay mode in Halo 3 and Halo: Reach  that allows for both playing and editing multiplayer maps.

Background
Forge is a game mode originally released in Halo 3 designed by Bungie to allow players to customize, save and share maps for Custom Games. Forge was originally created to be used by players to make relatively simple changes to maps in order to tweak their gameplay experience. Maps altered in Forge are referred to as map variants, and possible alterations include the modification of player spawn points, gametype objective points, weapon, vehicle and the addition of scenery items such as Crates, Fusion Coils, Grav Lifts, Teleporters, and various other items - In Forge players can add items that aren't available in unmodified maps. Halo 3 players proved so resourceful and imaginative with the tools provided to them that Bungie released two maps created specifically with Forge in mind. The scenery and layouts of these maps are almost entirely composed of movable and removable Forge objects. The Halo 3 Heroic Map Pack introduced which allowed players a huge amount of freedom in the creation of new maps. The Mythic Map Pack introduced, a massive map that is commonly referred to as the ultimate forging map in Halo 3, with three distinct spaces to Forge in, a wide variety of scenery to choose from, every weapon in the game and every vehicle in the game, except for the  exclusive vehicle, the Elephant.

Bungie announced that Forge mode will be returning in Halo: Reach.

In addition to functioning as map editor, Forge is also a play space. Up to eight players can be on a given map at a time, shifting back and forth between their chosen multiplayer model (Player Mode) and the Edit Mode model (a Monitor) by pressing Up on the D-Pad. A variety of Forge-specific settings can be altered by the party leader while in the Forge Lobby. As an example, players in Edit Mode can be granted more (or less) health and shielding as well as varying levels of Active Camouflage—editors can even be granted near or total invulnerability. There is also a setting that, if enabled, prevents all but the party leader from entering Edit Mode. Players can play actual games in Forge, experiment with Forge's features, kill each other, play diverse and varying 'honor rules' games, and do other things that aren't accepted in normal Multiplayer.

Halo 3 Forge
The tools in Forge in Halo 3 proved to be very primitive in terms of allowing players to edit maps. This is because Forge was created originally to allow players to make smaller changes to maps; Bungie did not expect so many creative and amazing maps to be created with the tools they provided.

Object Manipulation
In addition to modifying object spawn locations, Forge can also be used to change the properties of individual objects as well as different types of objects. Pressing X over an item allows a player in Edit Mode to alter that item's settings. Pressing X again allows players to view the settings for all objects of that type. Per-type settings can also be accessed by highlighting the given object's entry in the spawn menus, and then pressing X. An individual object (eg, a single Gravity Lift) may have its Respawn Rate (which determines how often it spawns) and its Place at Start settings altered. Types of objects (eg, all Gravity Lifts) may have their Run-Time Minimum and Run-Time Maximum settings altered, which allows mapmakers to ensure that there is always less than or more than a certain amount of a given item during gameplay.

Restrictions
There are some limits when placing items in Forge. The most obvious limit is the set of items permitted in a map—specific items are only allowed in specific maps. As an example, Hornets and Scorpions cannot be placed in The Pit, and Deployable Covers may not be placed in Sandtrap. (The technical explanation is that the tags for certain objects are only present in certain maps. Bungie likely did not place them for balance issues.) There are also varying limits on how many of each item can be placed; while you can usually place up to thirty-two Frag Grenades, you can only place two Scorpions when they are available on a map.

The amount of items you can place in Forge is also regulated by an economy budget system. Different objects are assigned to different values. This is to keep you from overloading maps and using an excessive amounts of memory. Deleting items that are already on the map will free up more money for other items. The spending limit is merely a counter to help the player gauge how much they can place on the map at a given time; no real currency is used. The highest encountered budget is located on the map Sandbox, with a grand total of 1,500 credits. However using the budget glitch, you can go beyond the limit set, to do this make the run time maximum one more than the current amount placed on map. or you can download canvases with the budget glitch already in.

All maps have a maximum limit of 640 items. The 640-item limit behaves somewhat oddly; removing items that are in the map by default does not free up more of the 640 "slots", so when editing large maps like Sandbox, it is best to move all default items to the side instead of deleting them and trying to replace them when you need them later.

Furthermore, Campaign objects, enemies, friendlies and “bots” cannot be added to map variants, and the basic geometry of the map cannot be altered.

Forge Filters
Forge Filters, referred to in-game as FX, are objects listed in scenery which came in the Legendary Map Pack maps, Cold Storage, and the Mythic Map Pack that change the camera settings. All the filters are as follows: Bungie has stated that the filters were added for better experiences in gameplay, making interesting screenshots, and for making machinima. The Juicy filter helps with seeing through the Energy Blockers, as does the Old Timey filter. Should more than one of these filters be spawned at one time, the effects will merge to create an entirely new effect. Combining all of the effects reduces visibility to the point where it is impossible to see.
 * Juicy - Everything is bright, vivid and colorful. Saturation is increased.
 * Colorblind - Everything is displayed in grayscale.
 * Gloomy - Everything is dark, shady, and somewhat desaturated.
 * Nova - Everything is extremely bright and blurry, though really dark shadows look quite sharp.
 * Old Timey - Everything looks like an old Western movie—the contents of the screen are displayed in sepia tones and are even given a flickering effect, making the screen resemble an old film projector.
 * Ink - Scenes are displayed as if they were drawn using an inkpen. Bright areas tend to look more like a grayscale version of the Nova filter.

Halo: Reach Forge
Forge mode returns in Halo: Reach as Forge 2.0, with a variety of updated tools to allow players to more easily edit maps, and of course with a Forge palette which has been updated to include items from the Halo: Reach sandbox. A map designed for players to create their own Forge Map Variants named Forge World will be shipped with the game.

Object Manipulation
Object manipulation in Forge 2.0 has been vastly improved.
 * When an object is being held by a player, that player now "orbits" around the object in their grasp rather than having the object "orbit" around themselves (the camera moves around the object rather than and not the object around the camera).
 * Players have the ability to select the physics of an object: normal, fixed or phased. Normal physics makes an object fall to the ground and be knocked around as in Halo 3. Fixed physics allows players to place an object and have it stay exactly where it was placed - even floating in the air. Phased physics is similar to fixed, but allows players to intersect objects with other objects or map geometry.
 * Players have the ability to "nudge" pieces, and the ability to edit an objects coordinates.
 * Players can alter the color of some objects to indicate team color.

Restrictions
Though Forge 2.0 is vastly improved over Halo 3 Forge, there are still a number of restrictions on what a player may do to edit a map.
 * The Forge budget returns to Halo: Reach.
 * The limit of number of a particular items placable in Forge mode returns, albeit with some changes. The limit now applies to categories of items; for instance number of walls now applies to walls as a category, with 50 wall objects placable rather than 25 single walls and 25 double walls.
 * Forge cannot be used to Forge Firefight maps.

Forge Filters
Some forge filters are limited to Halo: Reach only. Not all filters have been revealed, but below is a current list of the one that Bungie has confirmed.
 * Blue - Available in Halo: Reach only. It turns the environment Blue.
 * Purple - Available in Halo: Reach only. It turns the environment Purple.
 * Green - Available in Halo: Reach only. It turns the environment Green.
 * Next Gen - Available in Halo: Reach only. It makes the environment have slightly darker colors.

Trivia

 * The three most popular maps for Forge are Foundry, and Sandbox, most likely because of their large, open spaces, as well as the fact that you can put nearly anything on Foundry and Sandbox.
 * If you fire your weapon and then immediately enter Edit Mode, then upon returning to Player Mode, you may find that you've lost no ammo.
 * If you catch on fire (from a Firebomb Grenade or the Flamethrower) and then enter Edit Mode, you will take no damage. Upon returning to Player Mode, however, you'll find that the fire was "preserved", you will once again take damage.
 * The same trick can be used to delay the detonation of a grenade when you've been stuck. You can even delay the throwing of a grenade by entering Edit Mode immediately after pressing LT.
 * It also applies to any momentum that you've gathered while in Player Mode, if you jump into a Grav Lift or man cannon, enter Edit Mode, move away, and then return to Player Mode, you will fly through the air.
 * The Elephant cannot be spawned by editors due to its size, potential for lag, and specific purpose on Sandtrap, however more can be modded into the map.
 * Forge keeps track of stats and kills.
 * Created map variants can be saved and shared with the community. Also, if your creation is liked by Bungie, they will put it up to be downloaded by other players for use.
 * It is possible to do a glitch that can make a player have no weapon.
 * The idea of Forge was taken from Marathon: Infinity, which also has a map editor named Forge. However, there are multiple differences: Marathon's Forge was a separate application, acting as a 'pure' map editor in conjunction with another application, Anvil, which edited physics, sounds and shapes files. On the other hand, Halo 3 's Forge is merely a separate gametype that allows for the spawning and repositioning of objects.
 * Forge bears resemblances to Sapien, a program in the Halo Custom Edition Editing Kit.
 * Some weapons may be purchased at no cost if they were originally on the map and are not deleted. However this will leave you with a penalty making it so that you need to sell the same amount you have spent under zero to reach zero again and make money again.
 * Even when Edit Mode is restricted to "Party Leader Only", a programming mistake allows other players to enter Edit Mode through the start menu. (Patched)
 * If you stack both Elephants on top of each other and then put as many large objects (tanks) as you can on top the bottom, the Elephants will have reverse gravity.
 * Through a glitch, it is possible to wield weapons while in Edit Mode.
 * During lag or a glitch, it is possible to see the small blueish-white tear drop shaped orbs (item spawn points) that denote where an object has been placed while in Player Mode. When this happens, the spawn points can be interacted with while in Player Mode. If you shoot or melee them, you will hear the same sound that plays when shooting or beating a shield door.
 * The codename for the map making software for the first Halo game was called Forge before release.
 * When you are in Edit Mode, the reticule in the center of the screen appears to be the Marathon symbol. This is probably due to the fact that the eyes of the Monitors resemble the same symbol.
 * When in Edit Mode, the player can kill someone with a floating object and will get the Splatter Kill medal. If you do this on a Mythic map, you will earn the Tank Dropper achievement.
 * There is a mistake when editing options in forge, it says: View current options. Only the party leader may change options, regardless of what that person is the party leader or not.
 * It is unknown why you cannot place certain weapons on certain maps, but it is probably due to balancing. Vehicles would be unable to evade Missile Pods in indoor maps, and large vehicles like Wraiths and Scorpions might not even fit inside of indoor maps with small doors or rooms. However, this still does not explain the absence of weapons such as Fuel Rod Guns and Flamethrowers.
 * Forge 2.0 in Halo: Reach was originally going to feature Weather as a changeable object but was cut due to time restraints: "No weather effects in the Forge. Was something we wanted, but something we didn't get to in time."
 * In Halo: Reach, players will be able to change how objects interact with the environment with new effects like phased (goes through/into the environment and other objects), normal (acts like objects in Halo 3 forge), and fixed (stays in place, even in the air).
 * In Halo: Reach, Forge will be more precise. You will be able to put objects at certain angles and move them by a certain amount of centimeters, which will make buildings (forts, castles, etc) easier to build.

Internal

 * Forge World
 * Forge Objects
 * Forge Projects
 * Forge Tricks
 * Switches

External

 * Forge Demonstration Video
 * Bungie Article : "What is the Forge?"