Valve has made SteamOS the game focused operating system available for download. It's not the finished product, but rather an early beta version - and you should have decent knowledge of Linux commands if you want to give it a try.
Here are the minimum specs for the current beta version:
Intel or AMD 64-bit capable processor
4GB or more memory
500GB or larger disk
NVIDIA graphics card (AMD and Intel graphics support coming soon)
UEFI boot support
USB port for installation
If you're looking for more information head over to the Steam Community.