I've been using Dojo for a while. It has widgets with nearly the breadth of something like ExtJS, but is completely open-source. The documentation isn't all that great—thought it is improving—but the community is responsive and the code is well-written enough that it isn't difficult to figure out what the docs should say.

I don't know that I'd recommend it over jQuery if you don't need the widgets, but I don't have a lot of jQuery experience. The widgets were a big factor for us.