DJMala

Very cool and original 2D side scroller:

http://www.chroniclogic.com/gish.htm

I haven't bought the full game yet, but the demo is very cool. The main character is a ball of tar, and the game has a physics engine that allows you to do things like bounce, stick to walls, and squish yourself into small places. It's loads of fun and very different from anything else I've seen in the genre.