With the release of free Portal 2 DLC this week, Steam is featuring Portal 2 in one of their weekly sales.
Also on sale:
Portal 2 – 2-pack (one copy for you, one copy for a friend) $27.49
Portal Bundle (includes Portal and Portal 2) $17.49
And remember, you can download the complete Portal 2 soundtrack for free, through the Official Portal 2 website.