Wikipedia has an excellent table of JavaScript frameworks that compares the features of Dojo, Echo3, Ext, Google Web Toolkit, jQuery, midori, MochiKit, MooTools, Prototype & script.aculo.us, Pyjamas, qooxdoo, Rialto, Rico, YUI, and SweetDEV RIA.

http://en.wikipedia.org/wiki/Comparison_of_JavaScript_frameworks