Earlier this year Valve announced it was at work on bringing Steam over to Linux. At the time the company announced it would launch Left 4 Dead 2 on Linux first and then look into other games, but there has been little word since then. Yesterday, Valve created a new Linux blog to update everyone on the status of Steam on Linux. The good news is there has been a lot of progress made, as the goal is to get a "fully-featured Steam client running on Ubuntu 12.04." Left 4 Dead 2 has been ported over successfully as well, as it runs natively on Ubuntu 12.04. Valve is still tweaking L4D2 to ensure the Linux version runs just as well as the Windows version, and will update with the progress at a later date.
If you happen to run a different Linux distribution, do not worry as Valve is focusing first on Ubuntu and will then look towards other distributions. The company wants to make sure everything works perfectly on one specific distribution and will look at others depending on the success of the Ubuntu client. Valve will be updating the Linux blog fairly regularly, and you are free to email the team or leave comments on the blog. The company is encouraging communication from the community to make sure everything with the Linux version works how it should.