I'm surprised no one had mentioned jQuery in these forums yet:
jquery.com/
or better yet, jQueryUI:
jqueryui.com/
In situations where you want Convertigo to host the frontend of your mashable, but you want to change more of the UI than what is simple in CLP and CWC, jquery is an excellent toolkit for simplifying REST interactions from within JavaScript. Use a 'cancel' return value from a base transaction to return a custom XSLT page that provides your frontend. Then use jQuery to make subsequent requests to CLI or CWI transactions to get the data! jQuery also gives you shortcuts to parsing and selecting within the XML response from CLI and CWI.
jQueryUI includes the regular jQuery toolkit, PLUS some great widgets and skinning options for making an RIA type page.
