If you're into animation, as in cartoon and Anime, please take a look at Ktoon. KToon is a 2D Animation Toolkit running on Linux and Windows with a GPL license.

I don't cartoon much or do animations, but I have played around with this application and it rocks! It has all the features one would expect to find in such an application, like key frame editor, onion skin, drawing tools, and scripting. Please help these guys out and give them a donation if at all possible. I plan to do so even though I don't really use it.

For a walk through on creating an animation with KToon take a look at this tutorial. This tutorial is in English, but other languages are available on the Wiki.

Another 2D animation project I find really good, but a little buggy, is Synfig. This project has slowed, but is not dead. Synfig has Windows, Linux, and OS X builds available and is also open source. It really needs people willing to use it to report bugs, document it's features, maybe even do a little programming, etc.