Vivus is a little JavaScript class (little because it's lightweight and have no dependency) to make drawing animation with SVGs in a webpage.
Terry Gilliam shares his paper cut-out animation technique in a rare tv program from the 70s.
Timeline.js is a compact JavaScript animation library with a GUI timeline for fast editing.
Animata is an open source real-time animation software, designed to create animations, interactive background projections for concerts, theatre and dance performances.
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. Pencil is free and open source.