There are many similar frameworks to choose from. Such as: (in no particular order)
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]