If you haven't heard of Pencil you've been missing out. This software is great if you want to rough out an animation before creating it in 3D. Or if you just want to animate in 2D period. Its really a nice simple program that works as stated.
The developers say this of Pencil:
Pencil is an animation/drawing software for Mac OS X and Windows. It lets you create traditional hand-drawn animation (cartoon) using both bitmap and vector graphics. Pencil is free and open source.
Pencil is not intended to mimick web-oriented vector-based animation software such as Flash. Its main purpose is to make traditional animation. Neither does it try to rival commercial software targetting the professional animation sector. Pencil is intended to be a simple program enabling anyone to make 2D animation.
Give the program a try. Its fun just to animate a ball bouncing or a line dancing around.
The Pencil website is located here.
Download Pencil here.
The user manual is located here.