Draggable is a modular drag & drop library, allowing you to start small and build up with the features you need. At its most basic, Draggable gives you drag & drop functionality, fast DOM reordering, accessible markup, and a bundle of events to grab on to.
DOMtastic is a small, fast, and modular DOM & Event library for modern browsers. Same familiar API as jQuery.
Works great stand-alone or paired up with e.g. Backbone or Angular.