Valve will be releasing updates to the supported game list every Wednesday and you can check out the list now to see what's available.
On top of that you can get Portal free for a limited time:
Portal used to cost money. Until May 24th, it's free. End of story.
Well, technically speaking, there are some strings attached. Fortunately, they're entirely decorative. We just like the way they look, swaying in the gentle breeze created by a million people simultanously fainting from shock at the news that Portal is free. Now you have no reason not to try Portal.
Head this way to read more.
