General Purpose
MochiKit is a simple and compact Javax toolkit which provides basic signaling and asynchronous message handling as well as some visual effects, some of which are backported from scriptaculous.
GUI
Tibco General Interface shows that it is possible to create a complete visual IDE based on javascript and develop typical client-server style applications based on a browser.
Qooxdoo is like Tibco GI with less feature but more community spirit.