Mainly PC games...
Diablo II: I played online for 2 full years, and I still play from time to time
Vampire the masquerade: Bloodlines: I've beaten the game at least 20 times, will all the characters and playstyles, including unofficial add-ons
Fallout 2: Ancient game, but just too good to stop playing it (and so much better than fallout 3). I play it every 2-3 years
Warhammer 40k: Dawn of War: All expansions included
Farcry: For me the best shooter of all time hands down... Beaten at lest 4 times
Guitar Hero III (on Wii): Expert mode is so self-rewarding...
Super Mario 64: Just pure perfection
Advance wars: I've memorized all the enemy movements each turn...