Dan Webb compares four popular AJAX libraries (Dojo, Prototype, Yahoo UI, and Mochikit). Links are broken and the article is already more than month old 😉 but it's a good read. I ran into it while wrestling with Dojo toolkit which JotSpot uses and sponsors. It doesn't mention Google Web Toolkit (GWT) though which is understandable since GWT is a rather odd beast, odd in the sense it made me chuckle and shake my head.
Among the four, I like Dojo although trying to use it made me feel like I was wrestling a fat sweaty lady. Dojo, indeed. Doc! I am bleeding over here!