MochiKit.Animator
The release of MochiKit.Animator feels like quite a major thing here. You can read some comments by Kevin Dangoor and Ajaxian.
It’s a fantastic API, and it really looks and smells far better than Scriptaculous, the current king of the javascript animation space.
What’s really nice about MochiKit.Animation is it’s use of the functional style, which produces terse but expressive source that is often “obviously” right — much better than Object Orientation for a lot of tasks. Seeing this applied so neatly to animation is fantastic.