If you use Prototype, there's a very helpful, very new library of extensions at Scripteka.com (thanks to Ajaxian for the link).
Using Scripteka, you can browse extensions by tag, or view the latest or most popular extensions, all with links back to authors' sites and demos. The site itself also uses Prototype and has some nice examples of in-place refreshing.
If you develop web pages and don't use Prototype, check out Scripteka to see what you're missing out on. It's like adding a slice of cheese to your Vegemite sandwich...making a great thing even greater :-)
I like the fact that selecting a script from any link on the page finds and highlights it in the list. I also like the multi-select tag cloud that doubles as a simple search (all while staying on the same page).
Scripteka goes hand-in-hand with another useful site called Ajax Rain, which also collects scripts (including for other libraries like jQuery and Moo).
Tags: script, prototype, library, extension