I really like the look and feel of Pencil2D, and Pencil before that. So far it’s the best 2D animation tool i’ve seen for Linux. I have to be honest and say that i feel it’s still not something i can put my time and energy into using for anything major yet (for playing around, it’s great). It’s got some pretty big bugs, including a huge memory leak that prevents its use for HD animations, and is missing some basic necessary features. I continue to monitor its progress, continue coming here and helping other users, encouraging use and development of the program. Basically i see Pencil2D as having the potential to be the perfect application for digital 2D animation, on any platform…it just needs some work.
Of the applications i’ve tried, i think Pencil is easily the most enjoyable to use. Here are some others:
(I’m posting this with the assumption that you’re not looking for a vector animation program)
Tupi is vector, so if that’s an issue then it might not work for you. http://www.maefloresta.com
Blender has new animation features for its grease pencil tool, which allow it to be used for frame-by-frame 2D animation. Its grease pencil is not very versatile, though, so at this point i would say it’s limited to being used for sketching out rough animation. It has one major advantage over other open source programs, in that it has the ability to sync audio and video. http://www.blender.org/
Synfig Studio is working on getting what appears to be bitmap drawing included (via mypaint brushes), but from my brief experience with the dev version it seems to be not quite there yet. The previous versions are sloppy, in my opinion, but it seems to be popular regardless. http://www.synfig.org
Gimp has very basic animation capabilities built-in, and there is a plugin called GAP, or Gimp-GAP, which adds more animation features. In my opinion it’s a mess, though…very clumsy interface if you’re accustomed to working with any of the usual animation/video software programs. http://www.gimp.org/ (i don’t know an official website for GAP)
There is an animation fork of MyPaint, called Dopey. It’s still in a very early & unstable stage of development, but might turn into something good eventually. https://github.com/dopeyanimation/dopey
I’ve heard good things about Animation Paper (previously Plastic Animation Paper), but since i’m on Linux (and the free version doesn’t seem to work well in Wine or Virtualbox) i haven’t used it myself. http://animationpaper.com/