For myself, it's Worms Armageddon (1999, I think). I reinstall it every few years (usually after running into one of the vids online from the Let's Play WA tourney, which inspires me). It's still fun after all these years, though there's never anyone to play multi with. I'd play on WormNet, but the people still using that are gods among men and playing with so many tweaks and such that it's unrecognizable.

I also imagine I'll eventually reinstall CS:Source whenever I get a new computer again. (Assuming no HL:3 and new CS by then.) My current system is 7+ years old and dying. Just holding out til I can afford something new, and then will inevitably buy some new game for the first time in years.