CopperJS is a modular library aimed at making JavaScripting easier. It provides APIs for type handling, achieving certain design patterns, DOM/Events syntactic sugar, browser detection, dependency management, and a few add-ons.
Lithium is a lightweight library to be used along with jQuery. It takes care of software patterns like classical inheritance, observable (publisher-subscriber pattern), bind, namespace, IE8 JS 1.6,1.8 polyfills, and more, while jQuery takes care of the DOM.
Simple Javascript Image Viewer is an easy-to-embed Web based image viewer with which you can zoom using mouse wheel, pan using mouse drag, and preload images. It comes with complete keyboard support and extensions for things such as adding toolbars.
Stitch lets you keep your templates and pages neatly separated from each other in different files. It dynamically stitches the template and page together, and keeps the runtime overhead to a minimum as compared to most templating engines.