Spelunky.
A bit of a shame they didn't do anything special at the end. Just an "It's over!" screen, and on refresh everything was back to normal.
If the ever do this again, they HAVE to publish the list of games ahead of time (but maybe not what they're going to discount to), or at the very least, reveal a rolling list of the next few coming up as they go.
Also, they still really need a wallet system. Tiny little charges repeatedly make me worry my card is going to lock them out, and they can't be enjoying the extra transaction fees, anyway.