Here’s a nice site with some extensions for the Prototype javascript library
It cries for more contributions. But I think the main reason is that not a lot of people know that such a site exists.
So here’s the link, and enjoy the Prototype library.
And by the way, here’s a cool extension for Prototype developpers : Prototype Xtensions.
And some other cool articles/links about Prototype are available on Ajaxian.