Atmosphir
This is a rather interesting project from Minor Studios. Atmosphir: a build-your-own platform game, with a spiffy-lookin' level editing system. The video trailer after the jump shows some very pretty game worlds, and lots of point-and-paint editing of the levels. Shame about the awkward-looking central character - it'll be interesting to see whether you get to tweak the game physics as well as simply build the world. Atmosphir is currently undergoing closed beta, so you might want to try signing up for that if this kind of thing tickles your interest. Apparently set to be free, too.