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. While wielding an Assault rifle, the Chief could, with the flip of a switch, turn on a flashlight. The flashlight was most useful in levels Keyes and The Library where the amount of darkness made it difficult to see without the use of the flashlight.

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 MJOLNIR Mark VI (6) armor. His new armor was a great improvement on the old MJOLNIR 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 without it. The flashlight proved useful in the level High Charity.

Halo 3

It has been confirmed through Bungie that the flashlight will be in Halo 3. It has been confirmed by Bungie that the player will be able to it by pressing "UP" on the D-pad. This might also be how you use the Arbiters Active Camouflage.

Multiplayer

In Halo and Halo 2 multiplayer, the flashlight cannot be used.