Bootstrap, from Twitter is a heaping serving of awesomeness. In addition to it's easy to use grid system, layouts, good looking typography, and the fact that the whole thing is written in Less, it includes a handful of JavaScript plugins for jQuery and Ender to bring a lot of the CSS components to life. We're talking tips, popups, drop down menus and more.
For those of us who prefer MooTools, the JS included in Bootstrap isn't so useful. This library offers all the tools necessary to get the same functionality released by Twitter.
Included in this repo are numerous MooTools plugins that depend on MooTools Core, MooTools More, Clientcide, MooTools Behavior, and Behaviors for MooTools More. Building all these takes some configuration on your part (check out the MooTools Packager - here's a nice blog post on it by Ryan Florence), or you can just go to dev.clientcide.com and build it with the online tools there, which is what I recommend.
See: http://anutron.github.com/mootools-bootstrap/ for demos and code.