From Bit-tech.net:

Valve has confirmed that it is working hard on porting its Source game engine and Steam digital distribution service to Linux.

Rumours surrounding Valve's support of Linux have been doing the rounds for years, but little official information has leaked. While it's possible to get Steam and Source games running on Linux using the wine (Wine Is Not an Emulator) Windows compatibility layer, it's not ideal - and Steam, in particular, exhibits some odd behaviour when run in this manner.

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