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.