I was initially going to post this as part of my Portal 2 review, but it really deserved a post on its own. I was following the events from April the 1st on the user-created Valve ARG Wiki, which was frequently updated and an invaluable source of information about the events as they unfolded.
Just under two weeks before the Portal 2 launch date, Valve released a pack of indie games at a reduced price called the "Potato Sack". Within it, the games had been updated to include Portal related content. When players completed this content, they acquired potatoes (up to a total of 36) to display on their Steam profile.
Randy Lundeen, a developer at Valve, was emailed with questions about the Potato theme. He replied with cryptic messages, alluding to several real-world locations, which, when placed on a map and lines drawn between points, spelled "neilpot". This turned out to be a Steam group, the leader of which had screenshots which appeared to be of the first Portal game, but however showed Portal 2 co-op. These screenshots also had curious patterns shown on the walls — potentially belonging to a QR code.
This theory turned out to be correct, and a QR code was found pointing to this website, which showed how to get Portal 2 released early: the games from the Potato Sack had to be played.
Players successfully completed that task, and Valve released Portal 2 as promised... 8 hours ahead of schedule.