Mootools (www.mootools.net) is a promising young javascript animation library. It prides itself on being very light-weight and customizable (you pick the components you want even before you download). It has great animation effects, moreso than Scriptaculous.
Yesterday I started working with the library, and after just a short time I had something awesome looking up and running. I was using the slider effect to collapse and show comments on a blog post.
However, this morning, I decided to add the prototype library because I needed to do some AJAX work.