So Spelunky HD is out on Steam and I've been playing it like crazy. It's really fun especially when you start getting good at it.
The new version has an interesting feature - a "Daily Challenge." Every day, a level is generated that all players can play once, allowing people to compare their skills without any element of randomness. There are also personal, friend, and global leaderboards.
And yes, I'm making this thread so more people might buy the game and play those challenges. Currently it's only me and Celdur.
(If you want to see it, there are a billion and one let's plays for the challenges. Or you could just pirate it. I did at first but it just made me compelled to buy the game to access the online component.)
EDIT - Oh, if you liked the Binding of Isaac, you'll probably like this too.