Quake. I occasionally get an urge to FINALLY complete the first Half-Life game, but I'm at the forced crossbow part and ammo is scarce.
Also, if anyone here owns both the original Half-Life and Half-Life: Source, just before that part there's a little physics puzzle (similar to the airboat ramp in Half-Life 2). I definitely saw a HL2 oxygen barrel being used to push up the platform that I needed to get on. Was that puzzle always there?
EDIT: Oh yeah, Sonic 1-to-3, Sonic & Knuckles, and Knuckles in Sonic 2 and 3 (got the Sonic Classics Collection)
EDIT2: Oh, and also Destruction Derby for PS1 sometimes, Rainbow Six for PS1, and occasionaly me and my friend have a Tekken Ball marathon in Tekken 3, again on PS1