Banshee launch

Banshee Launching is a method to get out of Campaign or Multiplayer maps.

Rocket Launcher Walkthrough
Firstly, grab a Banshee and a Rocket Launcher, and flip the Banshee onto its back (do not flip it back onto its front). Then, stand on the tip of the wing of the banshee that is on the floor, aim the Rocket Launcher at the tip of the wing that is in the air and fire. The rocket will explode pushing the opposite wing tip down and your wing up. This will propel you into the air, sending you so high that you can see over the boundaries of some levels. The Fuel Rod Cannon is also an alternative for the Rocket Launcher on single player campaign.

Note: You do not need an Overshield to do this. However, if you do not have an Overshield, you must stand on the very tip of the wing or you will be killed by the explosion.

Note: This can also be done with those black shields on Relic and Turf. Overshield required.

Grenade Walkthrough
First, grab a banshee and some Grenades, and flip the Banshee onto its back (do not flip it back onto its front). Then, stand on the tip of the wing of the Banshee that is on the floor, throw the grenade at the tip of the wing that is in the air and fire. The grenade will explode pushing the opposite wing tip down and your wing up. This will propel you into the air, sending you so high that you can see over the boundaries of some levels. Note:Works better on Halo 2.

Note: You MUST have an overshield on.

Hammer Variant
Do the usual flipping procedure, then slam on the far wing with a Gravity Hammer. This sends you about as far as the grenade launch but is easier and doesn't require Overshield.

Scorpion Variant
This is almost the same as the Rocket Launcher one, but instead of a Rocket Launcher, we have a Scorpion.

Firstly get the Banshee on its back and get yourself on the wingtip, then have a teammate shoot the other wing with the Scorpion missile. This seems to get you flying vertically farther than the other methods, and as such can be found useful (or at least usable) in assault.

Note: An overshield is a must while performing this trick.

Extra Tip
To get super high off of maps, get into a Banshee and ascend to someplace above ground level (IE: The huge ledge on Headlong that looks like it should be out of the map but isn't) and do the Rocket Banshee Launch from there.

Halo 3
Most of these methods were, sadly, disabled in Halo 3. Bungie must have decided to take it out of the game for unknown reasons, perhaps for balance. There is one other way the Banshee Launch can be performed. Position the Banshee so that only the tips of the wings are touching the ground. Jump onto one of the banshees wings until you are on the bigger of the two rods, and stick the tip of the wing you are standing on with a plasma grenade. If performed properly, you will be launched incredibly high up. You MUST have an overshield for this to work.