Released in 2007, the "hybrid of FPS style and a new genre of spacial brain teasers" was acclaimed as one of the most original games of the year. According to Wikipedia, over four milion Portal copies have been sold since its release, not counting sales through Steam.
The game Steam page hasn't been updated to show the newly added Linux support so it won't show up in the Linux section yet, but this should change soon. Until then, to install Portal, search for it in the All Items sections or click the link below:
You'll obviously need to have Steam for Linux installed - for Ubuntu, you can get it via Ubuntu Software Center or by downloading the deb available on its website.
Below you can watch the official Portal trailer:
(direct video link)