"The Valve Linux team is proud to announce the launch of a limited access beta for its new Steam for Linux client.
Steam for Linux Beta client supports the free-to-play game Team
Fortress 2. Approximately two dozen additional Steam titles are now also
available for play on Ubuntu.
the Steam for Linux Beta client includes Big Picture, the mode of Steam
designed for use with a TV and controller, also currently in beta."
Download here (read notes below before running)
Notes (using Linux Mint Maya (MATE Edition))
*You will get a message saying it is a closed beta if you try to run the program normally. Type this into the terminal to run: steam steam://open/games
*Clicking "Big Picture" may cause the program to become unresponsive
*You can't actually download the Team Fortress 2. From what I have read
this is because you need a legitimate beta account. (Edit: Turns out you
can't play any Valve games using the hack above)