Vampire: the Masquerade - Bloodlines
I have over 40 complete playthroughs there and I doubt I'll be shelving the game for good any time soon.
But as for more replayable games, yes I'll agree with Burig - Frozen Synapse is incredibly good. I've recently started playing it, and it's really awesome. I'm sure I'll be playing it for a lot of time to come.
But there are some older games, I still enjoy, too:
Heroes of Might and Magic 3 (with the In the Wake of Gods mod) - Heroes has never left me. Without exaggerating, Heroes has been on a hard drive near me more than Minesweeper. At any one point, I can go and start a new game. I'm that accustomed that I don't need a "mood" or any "right feeling" or whatever.
Crimsonland (v. 1.9.8 to be precise) - a really fun topdown shooter. The point (in survival mode, which I play) is to, well, survive as much as possible employing a variety of weapons, powerups and even abilities against spiders, zombies, aliens and other critters.