There's a free/open source pixel editor (aptly named "pixeditor") written in Python that could use a new maintainer. You can see an old forum thread about it here.
It's stable and usable as it is, and has (in my opinion) about the most intuitive animation support that I've seen in any pixel editing program, but it's still in need of some more features.
Here's a relatively recent screenshot. Notice the animation timeline.