Came across qooXdoo and I love it.
Hard to choose a favorite widget as they’re all quite nice (Tree, Menu, Color Picker, Overlapping Window, Gallery, …).

It also seems that integrating them into apps is easy, so, I might give them a try sooner.

qooxdoo is an advanced open-source javascript based toolkit. qooxdoo continues where simple HTML is not enough anymore. This way qooxdoo can help you to get your rich web application interface done – easier than ever before.