Quake 2 - has a cool enough storyline, superior FPS gameplay. Quake 4 is good too.
What i'm playing right now: Gangland - an interesting strategy/tactics/whatever game, somewhat hard to describe, but it's fun.
If you don't mind going really oldschool - Celtic Tales: Balor of the Evil Eye - will need Dosbox to run, it's an open-ended turn-based strategy/tactics/RPG hybrid and it's reallllllly addictive, once you get into it. It has pretty, if low-res (for todays standards) 2d visuals.
Then, when you feel like wasting a ton of time, you can always play Sword of the Stars (with expansions). It's a 4x space strategy game, turn-based with real-time battles and it has a ton of replayability, just doesn't get old.