Flashlight

The UNSC flashlight is used in places that are normaly too hard to see. Most of the UNSC Assualt rifles have a flashlight attached to them, namely those of the MA5 series

Halo: Combat Evolved
In Halo: Combat Evolved, the Master Chief needed to get around the Ring mostly in the dark. With his trusty Assault Rifle, the Chief could, with the flip of a switch, turn on a solar powered flashlight. The flashlight was a very useful tool. Considering the amount of darkness in some of the levels, Master Chief needed the flashlight charged at all times. It was the key to victory many times.

Halo 2
During the events of Halo: combat evolved, Master Chief's armor was damaged. When he arrived at Cairo Station, he got the new MJOLNIRMark VI(6) armor. His new armor was a great improvement on the old Mark V(5) armor. the new helmet came with a improved flashlight. The new flashlight did not need recharging, It would turn on, and stay on, in mid, to total darkness. It would turn off automatically if there was sufficient light to see withou it. The flashlight proved very useful in High Charity.

Halo 3
The flashlight will no doubt return in Halo 3. If it will be the automatic or solar powered flashlight, is still unknown. It is also unknown what button will be used to activate the flashlight.

Multiplayer
In Halo and Halo 2, the flashlight was not a function. This proved both good and bad. The good part was that it would not give your position away. The bad part was that most of the Multiplayer levels were dark, and hard to see enemies. Halo 3 looks to be the same as Halo 2. There will be no flashlight in the Halo 3 mulitplayer. (Assumed)

The Arbiter
Probably the most frustrating thing about the Arbiter levels, is that a lot, if not all of them, were dark. Some levels were so dark, that the player had to rely on dead bodies on the ground to guide them to where they haven't been yet. They would also have to rely on running near the wall, to find a door, or corner. However, the lack of any proper lighting is balanced out by the arbiter's active camouflage.