Steam OS is a Debian-based operating system in development by Valve, designed to be the primary operating system for the Steam Machine game consoles.
Valve launched beta testing of Steam OS on December 13, 2013. Hence it does not have much pre-installed applications beyond web browsing and playing games, example: file manager & image viewer. Valve recommends Ubuntu for developing games.
Steam OS may or may not see successful adoption, but encouraged by the Steam OS initiative, gamers & developers will now start moving to Linux systems.