Facebook Animation Library exposes a powerful animation library which gives developers an easy way to improve their user interface with a line of code or two. All animations are CSS based, so a working knowledge of CSS will really help you out here. An open-source version of FBJS Animation that will work on pages outside of Facebook is available here.
jsAnim is a powerful, yet easy to use library for adding impressive animations to websites, without sacrificing standards or accessibility. Weighing in at just under 25 kilobytes, jsAnim packs a lot of punch for such a little application.