Have you been keeping tabs with Valve's ongoing Potato Fool's Day ARG? Ever since Steam started its Potato Sack sale, several indie games were updated with clues and references to Portal 2. On April 12, a couple of these games received new DLC content that added Portal 2-themed levels.
For owners of The Ball and Killing Floor, you'll pay a visit to a few sections of Aperture Science that have dilapidated over time. Toki Tori and RUSH players have new puzzles that feature Companion Cube and GLaDOS cameos. The most sinister of these updates is in the form of Amnesia's new chambers that players must survive, complete with recordings left by one disturbed lady.
If you're wondering what all this is leading up to, that's still not clear at the moment. Besides the recent DLC updates, doing certain actions within any the Potato Sack games triggers downloads for Portal 2 concept art and adds icons of potatoes to your Steam profile. One of the ARG clues hints at April 15 having something to do with Portal 2's release on April 19.