Today Valve announced the availability of Counter-Strike: Condition Zero (Beta) on Steam for Linux. The announcement was made on the official Linux Beta Access group page. It’s only been a short while since they released Half-Life and Counter-Strike Source so this is not a surprise.
Furthermore, a SPUF user (SPUF stands for Steam Powered Users Forums) found clues that 2 more Valve games are due to be released on Linux. He used some tricky arguments with the steam launcher and managed to play Half-Life 2: Deathmatch and Day of Defeat: Source. You can check it out on SPUF and YouTube.
His findings were confirmed by the Steam Application Database where entries for these 2 games were recently added (Half-Life 2 DM, Day of Defeat:Source) which usually means they will be made available soon.