Bioshock: Find an oxygen tank, cover it in proxy mines, pick the whole mess up with TK and toss into the nearest Big Daddy. Boom.
Also Bioshock: Line a long hallway with alternating trap bolts and proxy mines, then tag a Big Daddy from the other end with the pistol. Watch as Big Daddy alternately electrocutes and immolates himself trying to get to you, only to (hopefully) fall dead at your feet.
L4D: Use a Smoker to yank one player from around a corner (doable with careful edging), then have your buddy tag them all with Boomer bile as they rush forward to attack you. Sit back and laugh in respawn mode while the zombie horde + your two Hunter friends rearrange their faces into something only Salvador Dali could love.
TF2: Rocket jump off the head of an enemy coming out from under the bridge canopy in 2fort, land on the bridge canopy. Bonus points for doing this to a scout trying to doublejump into your base.
UT2004: Rocket reflection with shield gun secondary fire. Almost impossible to do, but confuses the ever-loving fuck out of your enemy.
Any UT game: Get off a lethal shock combo by piercing your opponent's plasma balls. Hilarious and Freudian, all in one glowing purple package.
Smash Bros.: Kill enemies as Link (or Young Link) using *nothing* but bombs.