Jitsu is a next-generation user interface toolkit for building rich web apps

Jitsu is an open-source Ajax toolkit and framework that’s been in development for a couple of years. Unlike most, it uses XML as the presentation language, and I believe it’s unique in providing a compiler for the XML. It works in all modern browsers and has the usual full range of user interface controls and widgets for web interfaces. The site has plenty of documentation, a free download, and a slew of demos available.  It uses Microsoft’s .NET framework, so much run either on a .NET server or a Mono server (Unix port of .NET).