More jQuery plugins, about 3-4 thousand:
http://plugins.jquery.com/.
And there is even more when you just google what you are looking for...
I do like MooTools, jQuery and Prototype, but they all do different things and are not strictly comparable. jQuery is a toolkit that is based on DOM manipulation (and is pretty damn good at it) while Prototype framework is a basis for other frameworks/toolkits because it extends the basics of Javascript objects, arrays etc. MooTools in the other hand is nicely built modular system that you can expand when needed and it can be used to do what ever either Proto or JQ are doing.
I would recommend that you read
this. It sums it up pretty nicely.
And for useful stuff for bbgs, I'm making my own js library. I after I finish the code comments/api I can post it in this site so you guys can comment it

I feel like that the other frameworks/libraries/toolkits are bloated, and you really don't need all of their options.