Source mods. They're free, I can run them, and they're always well-populated no matter how shitty they are.
I've actually been contemplating writing a series of Source mod reviews, considering there are so fucking many. But then, I rarely read game reviews anymore, so it'd be hypocritical of me to write my own and expect anybody to care. (I feel the same way about any fiction ideas I have, as well as blogging in general, incidentally.)
It is too bad that 90% of all mods are about zombies. As neat as slaughtering waves of the undead is, there is not much room for strategy.
Current Source mods I have: Hidden, Zombie Panic, Zombie Master, Garry's Mod (and accompanying serverside zombie mods), Dystopia, Fortress Forever, SourceForts, Synergy. A few of these were played for about an hour.
Aside from that, into Call of Duty 4, DotA, tried to get back into Splinter Cell: Chaos Theory and Battlefield 2 recently and rediscovered why I gave them away in the first place.