Valve has announced
SteamOS, a Linux-based operating system built around Steam and "designed for the TV and the living room".
Update #1: the second announce has been made and it's about
Steam Machines. A beta is on its way to design and test a box produced by Valve and running SteamOS.
Update #2: the third and last announce is now revealed, it's the
Steam Controller, an enhanced and hackable controller. As for the
Steam Machines, a beta program has been launched.