Half-Life 2 and Half-Life 2: Episode 1, Half-Life 2: Episode 2, and Half-Life 2: Lost Coast have been in beta form on Linux since early May while Half-Life 2: Deathmatch has been in beta on Linux since March. After much testing by Linux gamers, Half-Life 2 is now deemed by Valve as stable on Linux.
Via SteamCommunity.com for Half-Life 2, "added support for Linux operating systems," in the game's public release. This Half-Life 2 update also adds in support for the Oculus Rift VR (Virtual Reality) system too and now works with SteamPipe for optimized content delivery.
Already this week out of Valve we have seen the Source SDK 2013 release that now supports Linux and Portal went out of beta on Linux. Expect more good Valve Linux news in July.