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.
The beta version of Left 4 Dead 2 is the same size as the main game, so you have a hefty download ahead of you. (I assume the same is true of Portal.) Steam for Linux officially launched in February, so it hasn't taken too long to get Portal and L4D2 on there. Half-Life 2 next?