CoD4: Leaving claymores on random street corners where you don't expect them. Sneaking up behind a sniper, planting a claymore behind him and running away fast. Calling an airstrike on Shipment.
L4D: Smoking someone in a spot that their team-mates can't reach, like the bit just before you fall down into the field in Blood Harvest. Smoking/pouncing people off of high drops so they die instantly. Locking a douchebag player out of the safe room/downing him with my hunting rifle before he can get in.
Soulcalibur IV: Ring-out grab at the start of a match. Using nothing but one move repeatedly against Yoda (this actually works. he's a terrible character). Kicking a person from my Player Match room for having a shitty custom character.
Street Fighter IV: Using Sagat online. Using Sagat offline. Using Sagat to perfect Seth on Hard three rounds in a row. Using Bison's Nightmare Booster on someone who had no health left. Going a whole match using nothing but Akuma's Aerial Gou Hadoken.
Gears of War: Stood beside a downed enemy until he bled out. Waited until everyone from both teams was in the central area of Gridlock, then firing the Boomshot in there, killing everyone except me and winning the match.
Gears of War 2: Sticking a grenade/smoke/ink on a downed enemy. Picking up a downed enemy just so I can break his neck. Taking the Torque Bow just because some whiny asshole wants to use it and tells everyone not to get it, then just using my Lancer on purpose.
Red Faction: Using the pistol.
Red Faction Guerrilla: Using the Rail Driver with a Vision Pack. Using the Enforcer or Arc Welder with a Jetpack. Sticking remote charges on my own team's control points in Damage Control, so I can set them off to kill the enemies when I see that the point is under attack, then just run in there with the Reconstructor and repair it.