If you're a glutton for punishment and have a good tablet you can animate surprisingly well in Gimp. It's basically traditional animation with digital assistance. X)
For example: http://spartydragon.deviantart.com/art/Chicken-Doom-126746505
I did most of the drawing on this in Photoshop Elements because I didn't realize Gimp's animation playback was so good (And Element's playback so crappy/fussy) but it wouldn't be that different.
The only downfall is that neither of those programs export to flash. :/ WHICH is why I downloaded Pencil: http://www.pencil-animation.org/ It's a free, open source, no frills animation program that exports to flash. I only messed with it for a little bit before I had to return the tablet I borrowed but it was pretty good. 
