The Plasma Repeater from Halo: Reach.
The most unique feature of the Plasma Repeater is that it actually gets more accurate as it continues to heat-up. The key to using this weapon effectively is to fire it so that it maintains a high temperature without overheating.
The Plasma Repeater, unlike the Plasma Rifle, actually does not gain heat or overheat while shooting it. Instead, the firing rate goes down, making shots slower and slower, eventually (if you keep shooting for no reason and don't die) it will slow down so much it would be 1 shot per 2 seconds. However, there is a "vent" function which allows the Plasma Repeater to continue firing at it's normal rate. The Plasma Repeater plays an essential role in assaulting a base, and preferably for close combat.