Here is a list of nice javascript libraries that were built on top of the prototype javascript framework:

- table sorting by Andrew Tetlaw

- Outstanding and really easy form validation by Andrew Tetlaw

- lightbox modal window by particletree

- a variation: TinyBox that is much smaller, by Alexei

- scrollbars by Patrick Ryan (Agave Group Design)

- another scrollbar script by jools.net (requires script.aculo.us)
- splitpanes by Jools.net again (requires script.aculo.us)

I’ll update this list whenever i find another useful library. Feel free to suggest additions via the comment form.