Each of the above has their strengths and weaknesses, and there are many more frameworks than I can list at the moment, but even with the select list above we can see there are many more contenders in this space than just Cappuccino, Sproutcore, and GWT.
GWT (http://code.google.com/webtoolkit/)
SmartClient (http://www.smartclient.com)
NOLOH (http://www.noloh.com)
DHTMLX (http://www.dhtmlx.com)
QooXdoo (http://qooxdoo.org/)
Vaadin (http://www.vaadin.com)
Echo (http://echo.nextapp.com/site/echo2)
Servoy (http://www.servoy.com/)
Sproutcore (http://www.sprotcore.com)
Sencha (http://www.sencha.com)
Each of the above has their strengths and weaknesses, and there are many more frameworks than I can list at the moment, but even with the select list above we can see there are many more contenders in this space than just Cappuccino, Sproutcore, and GWT.
Disclaimer: I'm a co-founder of NOLOH
[edited for clarity]