I really liked the Bullseye in Resistance: Fall of Man
It was essentially just a machine gun, however its secondary fire would 'tag' an enemy, meaning all bullets fired would track the tagged enemy around corners etc. Pretty much just like the gun out of The Fifth Element.
In fact lots of the weapons in the Resistance franchise were awesome. The Auger was a gun that could see and shoot through walls, but also could deploy shields that only it could shoot through, the Rocket Launcher had a secondary fire that launched a cluster of razor blades if enemies got too close, the pistol had a secondary fire that would detonate its bullets (so you could pump a strong enemy full of rounds, then detonate them all in one go for an awesome gib-fest)