Trine 2
I didn't really understand the "pre-order and get early beta access!" concept until it happened for a game I actually want to play. I was going to buy Trine 2 regardless -- my co-op experience with its predecessor was well worth the price. So pre-ordering it was a no-brainer, and as a bonus, I got the opportunity to play through what is effectively a beta-quality demo.
Which is pretty great, by the way. The snake boss at the end was a confusing encounter, and the overall experience is still a little unfinished (chiefly, the thief's controls feel a little floaty ... although maybe they're supposed to). But it's already as good as the first Trine, and I really can't wait to get my hands on the rest of the game.
Progress: Finished the beta levels