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...