What is your favorite Javascript UI framework out there ?
coffee_dregs: For information rich applications (ie. B2B-ish), definitely ExtJS.  Pretty steep learning curve since, unlike jQuery UI, you sorta have to take all of nothing.  We use ExtJS everywhere where using data/information is more important than look-n-feel. Put together CoffeeScript (http://jashkenas.github.com/coffee-script/) with ExtJS and you'll be in RIA dev heaven.  Fast and very productive.For applications where look-n-feel, customization, etc is more important (ie. B2C-ish or consumer internet), HTML/CSS/jQuery is the way to go.