Raphaël is a small JavaScript library that should simplify your work with vector graphics on the web. In case you want to create your own specific chart or image crop-n-rotate widget, you can simply achieve it with this library.
ShareJS is an Operational Transform library for NodeJS & browsers. It lets you easily do live concurrent editing in your app.
Mapstraction is a library that provides a common API for various javascript mapping APIs to enable switching from one to another as smoothly as possible.
Zotero [zoh-TAIR-oh] is a free, easy-to-use Firefox extension to help you collect, manage, and cite your research sources.
Sarissa is a cross-browser ECMAScript library for client side XML manipulation, including loading XML from URLs or strings, performing XSLT transformations, XPath queries and more.