Flashlight

Flashlights are quite common in the UNSC military forces for combat in darkened areas. As such, many UNSC weapons have mounted flashlights below the barrel. In addition, all known variants of MJOLNIR armor have an inbuilt flashlight on the helmet.

Halo: Combat Evolved
In Halo, the flashlight was originally intended to be only usable while carrying the MA5B Assault Rifle or M90 Shotgun, both of which have inbuilt flashlights mounted beneath the barrel. However, this was eventually changed. The only flashlight usable in the game is that mounted on the right side of the MJOLNIR helmet. This change was incomplete at release, as the flashlight's beam will still swing with the gun's movement while melee-attacking. Marines can be seen to use the gun-mounted flashlights on the levels Truth and Reconciliation. The MJOLNIR armor's flashlight has a limited battery pack which will slowly deplete while in use and recharage when turned off. The flashlight turns off when it has been activated for around a minute.

Halo 2
The MJOLNIR Mark VI armor also contained an inbuilt flashlight. However, this model was more advanced, and drew its power directly from the suit's power source. Because of this, it did not have a "battery" and could be used for any length of time. However, it had a light-sensor on it which caused it to turn off in well-lit areas. Although the assault rifle was absent from the game, the shotgun still possessed its flashlight. Despite this, it could not be used, and no Marines were ever seen to do so.

Halo 3
Since no major changes have been made to the MJOLNIR armor since Halo 2, the flashlight is expected to function in much the same way. Like in Halo 1, both the assault rifle and shotgun possess inbuilt flashlights. However, the shotgun's flashlight is now mounted on the weapon's left side rather than under the barrel. It seems likely that neither of them will be usable and that the MJOLNIR armor's flashlight will continue to be the only one usable in-game.

Multiplayer
The flashlight has never been usable in any form of multiplayer game. Why this is was not clear in Halo, but in both Halo 2 and Halo 3, the same button is used for voice chat in multiplayer. In Halo 3's Forge, the same button will be used to turn into a Monitor in order to edit the map, thus confirming that it will not be usable.