I've enjoyed using these two :)
http://www.zefrank.com/scribbler/scribblertoo/
http://al.chemy.org/download/
Alchemy's a little more complicated to use, but it's a great deal more versatile. Both programs are fun for playing with ideas. The mirror tool on alchemy is reeeeeeally fun for drawing faces :)
I'm making slow progress on a few different projects for uni right now, so in the meantime I'm posting doodles, sketches, stories, ideas - so I stay relatively sane.