As of this week, Valve Corporation's Steam service is officially available to Linux users. The added OS support was announced in 2012, and an ongoing beta test of the new version transitions now into a proper release. Valve's Team Fortress 2 is also now officially supported by Linux, and those who grab it for the open-source OS will, until the end of February 2013, receive Linux mascot Tux as a free in-game item.
The launch of Steam for Linux also brings with it a sale on all Linux games that will run until February 21 at 10am PT. The list doesn't include many of the bigger games, but popular hits like FTL: Faster Than Light, Bastion, Serious Sam 3: BFE, and various Valve titles are all available. That list will only grow over time, but it's a good start.
The Linux-supported Steam client is available now as a free download from the Ubuntu Software Center.