Having seen a few complaints from fellow GONers about how hard it is to find good stuff to upvote on Steam Greenlight (most notably in the comments on this article), I thought I'd make this thread as a point of reference to get people started.
First off, I recommend Pinball Arcade by FarSight Studios. It's already available on iOS, Android, 360 and PS3 if anyone's interested in checking it out beforehand (the PC version will most likely be a port of those). FarSight have already successfully funded two Kickstarter campaigns to get the licenses required for adding particular pinball tables into the game (first one was The Twilight Zone, second one was Star Trek: The Next Generation).
Greenlight link: Pinball Arcade
Next up, Postal 2 Complete. This is an official Running With Scissors Greenlight campaign (check their website to verify) and after the disaster that was Postal 3, it'll be a nice change to get back to the "so bad it's good" days with the re-release being done completely in-house (believe it or not, Postal 3 development was outsourced to a Russian company). There may even be a real new Postal game in it if enough revenue gets raised from sales of this if it's greenlit.
Greenlight link: Postal 2 Complete
EDIT: Don't be shy. Jump in if you've found something good that you think should be upvoted. This is a collaborative effort.