Left 4 Dead 2
Once again, Valve is offering early access to a game if its fans jump through a hoop or two. This time it's Left 4 Dead 2, and the offer is a piece of the Cold Stream downloadable content. A blog post outlines the details.
The developer is currently in the testing phase, and is offering an incentive for the community to become bug-finders. If 20,000 people earn the "Stream Crosser" achievement by the end of the day on Sunday, July 10, Valve will release Dead Air (from Cold Stream) on July 22. Stream Crosser is awarded for surviving a campaign of Cold Stream, so it shouldn't be too unobtainable. If you don't own the game yet, it's just $5 until 10 am PST today.
Valve previously offered early access to Portal 2 in exchange for playing indie games. This netted users the anticipated game about 9 hours early. Unlocking DLC for an existing game probably isn't quite as exciting, but early is early.