Valve is currently testing Half-Life 2: Deathmatch on the Linux Steam client. So far it runs flawlessly minus a few graphical (mostly stutter) glitches. Grab a copy for $4.99 in the Steam store.
Fast multiplayer action set in the Half-Life 2 universe! HL2’s physics adds a new dimension to deathmatch play. Play straight deathmatch or try Combine vs. Resistance teamplay. Toss a toilet at your friend today!