There's now more than one thousand native games available via Steam for Linux / SteamOS.
As of writing the article this morning in the US, there's 1,002 games available for SteamOS + Linux as shown here. If including other software and demos/DLC, etc, there's 1,853 items available natively for Linux.
Yes, Valve's Steam is trying to make Linux a more gaming friendly OS. They also want more game buyers from Linux users who want to play game on Linux rather than Apple Mac, or Windows.
Linux is also free, and open source, so Steam could make changes to Valve's Linux OS without getting sued if they use a non-free OS. The OS being free would also make the Steam Machine cheaper because Valve, and Steam Machine buyers won't have to pay for a Windows License.
