Valve has released the beta client for Steam for Linux. Team Fortress 2 comes included with the client download (it is free-to-play after all), but if you want to play some other games, you'll be pleased to know that there are a total of 26 Linux games now available on Steam. Most of these titles are indie games, but there is one full-fledged retail game Serious Sam 3: BFE - that sticks out from the bunch.
At the moment, the Steam for Linux beta is only compatible with Ubuntu 12.04 and above. Steam for Linux team member Frank Crockett says that Valve will eventually release Steam for more Linux distros. The beta for the Steam for Linux client also supports the Big Picture mode, also currently in beta.