Diablo II !

I don't want to imagine how many hours I spend playing this games (actually I do, because every single second was worth it because this game is so awesome !), probably only rivaled (time-wise) by CS and WoW (I'm cheap, so I had to invest as much time as possible into it so I wouldn't feel so bad about spending 12€ each month).

It's also the only game I have installed 3 times on my PC