ARGLBORPS  This is an incredible package. I've been using since the days the application was still called "Moho". Not only can you use bones to do 2D animation, it also let's you use 2D objects in 3D space, you can do blurring even of vector object, mix in bitmap object and use particle emitters to simlate smoke, swarms, snow etc.

It also comes with tools for lip synching. I have even used it for some motion graphics as a "poor man's After Effects".

For the money this is a really great package, it's very stable and you can render your animations to movies in batch process mode.

The only gripe I have with it that the UI and the way it functions is quite unusual, so you really have to take some time to get used to the programme.  
(Version 6.0)