While I consider myself proficient in a variety of JavaScript libraries like Prototype, jQuery, and YUI, my library platform of choice at the moment is MooTools. I’ll evaluate each project on a case by case basis and each trial will be different, but I’ve found MooTools to do an adequate job for the type of projects I work on.

Web Design Ledger posted a good collection of user interface enhancements using the MooTools library. It’s got a few good tutorials that could probably be implemented in your next project.