Natural Selection 2 is a multiplayer first person shooter and real-time strategy game by Unknown Worlds Entertainment. The game has been around for the past year on Windows and is a visually impressive title.
I've received a report today from Phoronix reader "Nikola" that Natural Selection 2 has appeared in the Steam Linux library and is installable. It's appeared in the SteamDB as of this morning. Update: The game can be downloaded now via the Steam Linux client but the official release is planned for 30 August.
The game is available via Steam for $25 USD.