http://www.skulltag.com/
New version of Skulltag, a mod for Doom 2. Has bots, lots of new maps/game modes, and piles of other crap in it.
http://www.experimentalgameplay.com/
A bunch of students are doing an experimental games project. Each week, they release one new unique game made by one person in seven days. Their goal is to keep doing this until they have 100.
Already has a bunch of weird games up.