A company called Sencha do a lot of impressive work when it comes to the web. Check out Sencha - JS Framework and you'll be impressed.
They claim to be the First HTML5 mobile web APP framework and the allow you to save copious amounts of development time while producing code that works across multiple plantforms, ah the coders dream!
On the Sencha site you'll find demos, an overview, the regular testimonials, road map & licensing. They also have info about training & support as well as community components like forums so you can share advice with fellow web developers.
The sorts of apps you can produce (in the browser) are fantastic and it's all thanks to CSS3 & HTML5, and of course you have to give props to Sencha for developing the Framework to be able to execute effectively and efficiently.
Sencha provide both commercial & open source licences so they're pretty flexible. They also have a number of great products (they acquired ExtJS when started off as a spawn of the YUI Library).
Although there are plenty of Javascript Libraries around Sencha have the swiss army knife of products and clearly demand your attention when you're next revisiting which JS Framework you'll develop with.
