I've used ExtJS and while its a bit heavyweight for many typical sites, if you're building a very rich complex UI it can be very powerful. Where it really exce

I've used ExtJS and while its a bit heavyweight for many typical sites, if you're building a very rich complex UI it can be very powerful. Where it really excels is in support for IE6-8 - all of their controls, code etc just works, which is a godsend if you need to support these browsers.