The Flashlight is an item cut from Half-Life: Blue Shift seen in an early Trailer. A flashlight is actually available in the final game, but it is used as a separate entity from the normal weapons.
The Flashlight would probably have replaced the HEV Suit Flashlight from the original Half-Life. Just as a normal flashlight, this would enable players to see in the dark with a light wave which will allow light. Though, unlike the other Half-Life games, this flashlight would have been a hand-held item. It is still seen being used by some NPCs.