I hear a lot about ExtJs actually. I've been using their framework for complex Javascript systems for a while now and it keeps getting better and better. I can see they invest a lot of resources in it. Its not popular with the 'gimme it for free' crowd though. I agree its not for everyone though its a commercial project (albeit free with GPL licence for open source projects), but then again you do get what you pay for here - nothing else that I've seen so far (and I've seen a lot) even comes close in terms of polish. Another common extjs deterrent is that its a complete framework with all batteries included, and requires an investment in a new mindset as far as how to develop single page browser apps. I found it more similar to Java Swing apps or other Desktop tool kits than any other JavaScript framework.