Several Phoronix readers have been writing in this afternoon that The Witcher 2 has appeared on SteamDB.info today with a "The Witcher 2 Linux Beta Retail" entry. It's not yet publicly accessible for Linux gamers to play, but the Linux listing for Steam indicates that it may be on the way very soon.
Witcher 2 was released in 2011 as a successor to The Witcher from 2007 as an action, role-playing "hack and slash" video game. Planned for release next year is already The Witcher 3: Wild Hunt.
Witcher 2 is powered by CD Projekt RED's REDengine, which i known to be supported on Microsoft Windows, Xbox 360, and OS X (thus an OpenGL renderer there) and the next-generation REDengine 3 will also support the PlayStation 4 and Xbox One.
More information on Witcher 2 can be found at SteamPowered.com where it currently costs $19 USD.