http://www.steampowered.com/nvidia/
If you happen to have an nVidia graphics card (and 2/3rds of people seem to, according to various stats) then you too can download the free Portal: First Slice!
Just go to the link above, it will check your machine to make sure you are running an nVidia card and then you can download and play Portal! (Windows only, I'm afraid).
Really worth a look, a fantasic game! More about it on our next podcast, too!