You might want to check out
'Yahoo! Widgets' widget engine. Widgets are little modules that are placed on your desktop and give you access to little controls/pieces of information. You can check your free disk space, see your local weather forecast, tune into online radios, launch applications, read RSS feeds, etc. They might not provide any great additional functionalities, but they sure are good to look at (remind me to talk about 'The Chocolate Wrapper Theory').
I'd encourage you to check out the coding for these widgets. They provide a good and easy to understand example for XML, which seems to be the de-facto language for next-gen data representation and transport.
Windows Vista will have its own set of widgets in a 'Sidebar'. I guess with a widget engine built into the OS, the days of Yahoo! Widgets are limited.
If you really have the time, check out
Kapsules, an alternate widget engine for Windows.