Sidr (GitHub: artberri / sidr, License: MIT) by Alberto Varela creates menus that look like the sidebars found in recent iOS apps. It can cope with multiple menus on a page, and can load content remotely. It's also responsive, so it should work well in mobile projects.
The author has written up documentation complete with demos, and has included a Grunt build script. It seems like the exact sort of UI component that the next great web-based RSS reader might use...
Huey (GitHub: michaelrhodes / huey, License: MIT) by Michael Rhodes will find the dominant colour of an image and return it as an RGB array. This is all performed client-side, and doesn't even depend on jQuery. It could be used to create the kind of effect seen in iTunes, where the background colour changes to suit the selected album art.
Backbone.Advice (GitHub: rhysbrettbowen / Backbone.Advice, License: MIT) by Rhys Brett-Bowen is a Backbone plugin based on Angus Croll's advice pattern. It basically adds functional mixins to Backbone objects, and can be wrapped like this:
Rhys sent in a whole bunch of other Backbone-related projects, including Backbone.ComponentView and Backbone.ModelRegistry. Backbone.ComponentView is based on goog.ui.component from Closure Library, and also works with Backbone.Advice.