KToon is a 2D computer graphics animation toolkit designed by Colombian Toonka Films for animators. (GPL)

Pencil is an animation/drawing software for Mac OS X, Windows, and Linux. It lets you create traditional hand-drawn animation (cartoon) using both bitmap and vector graphics (GPL)

Synfig is a powerful, industrial-strength vector-based 2D animation software package, designed from the ground-up for producing feature-film quality animation with fewer people and resources. (GPL)