HLDM and a bit of Worms is always good. Gabez loves the Mario Castle lobby!
I'd also like to take this oppertunity to urge all to upgrade and get Half-Life 2 as quickly as possible. Without trying to overstate the game, HL2DM is BEST*.
* When there's like, 8+ people on the server it turns kind of crappy, but it's great otherwise!