Google have released their Widget Library to help developers make Web 2.0 applications faster and easier.
The toolkit is the standard Google kit used to make GMail, Google Groups etc
Plenty of online demos. First impression is that they look very basic but as you play with them and start thinking how you would do it, you begin to realise the power of them.
May look into this toolkit for an upcoming Web 2.0 app Peter and myself are planning...
The fact that they are cross-browser with no loss of functionality is the best bit. They also have a nice tool that allows you to write your AJAX apps in Java and they have a tool that runs through and converts your code to cross-browser compatible DHTML.