M319 grenade launcher

The Unidentified Grenade Launcher, comically known as the "Pro Pipe" by Bungie employees, is a single-shot explosives launcher in use by the United Nations Space Command during the Human-Covenant War. It was used during the Battle of Reach. It is likely to make an appearance in the upcoming game from Bungie, Halo: Reach

Description
The grenade launcher has two firing modes. The first launches grenades at around normal grenade speed, which detonates on impact with an opponent or after normal grenade time after hitting a non-opponent surface. The second firing mode shoots a grenade that explodes when the player wishes. This is performed by holding down the trigger, which fires the grenade but it is merely primed, it will only explode after the trigger is released. This enables the player to lay a trap and essentially creates an explosive with a dead-man's trigger (a trigger that will go off once it is released, e.g. when the player dies).

Also, the grenade acts as a form of EMP that could bring down or temporarily disable vehicles, but only when used in the firing mode that allows the player to fire the grenade in a dormant, primed state.

Trivia

 * It appears to be roughly based off of the M79 grenade launcher, a weapon used throughout the Vietnam War. This is due to its single-shot break-action configuration.
 * The functions of the grenade launcher are similar to that of the Demoman's grenade launcher in the Valve multiplayer game Team Fortress 2.