Jon Udell recommends Macromedia's Flex presentation server to build rich Internet apps:

After a decade of web-style development, I’m sold on the idea of using markup languages to describe the layouts of user interfaces and to coordinate the event-driven code that interconnects widgets and binds them to data. The original expression of that model was HTML and JavaScript, but variations have flourished.

Mozilla-based applications have been using XUL (XML User Interface Language) for years. The Laszlo Presentation Server uses a description language called LZX. Now Microsoft has previewed XAML (Extensible Application Markup Language) for Longhorn.

Now comes MXML (Macromedia Flex Markup Language), the latest development in Macromedia’s ongoing quest to reposition the near-ubiquitous Flash player as a general-purpose presentation engine for rich Internet applications.

As with the Laszlo product we reviewed last fall, Macromedia’s Flex is a Java-based and XML-driven presentation server. You can deploy it to an existing J2EE server on Windows, Linux, or Solaris, or use the included JRun server. Nothing about Flex inherently requires a J2EE environment, however, and Macromedia is working on an implementation for .Net, too. But for the time being, it only supports Java app servers.