Valve has thrown a bit more of its weight behind Linux with the release of beta builds of Portal and Left 4 Dead 2. If you own either (or both) games you should find that beta versions have materialised in your Steam library - along with Portal 2, according to some. The Linux build of the first-person puzzler has so far gone unheralded by Valve, but here's a blog post describing the latest beta version of Left 4 Dead 2. In addition to letting Linux users play Valve's zombie hit, the download acts as a "testing ground" for its new Extended Mutation System. Thankfully, you can try the beta on Windows and Mac too.
Steam for Linux
Valve are seeking "experienced Linux users" to test the Linux version of Steam. This will be the test that Valve talked about back in September, which means that the Linux Steam client, one Valve game and "support for Ubuntu 12.04 and above" will be extended to 1,000 lucky applicants who manage to survive this survey.