There 11 Sidebar Gadgets that come baked into Vista. Some basic ones like a photo Slide Show, Calendar, Clock, CPU Meter, RSS Feed Headlines, Weather and a couple more. But, what if I want to create one that offers different functionality?
How can I create a custom Sidebar Gadget?
It's actually rather easy to do. As long as you are comfortable with HTML, JavaScript and CSS with splash of XML.
The Gadget Development Overview article in the MSDN library explains how easy it is to create a simple Hello World gadget.
There are also some other resource links listed at the MicrosoftGadgets.com website.
The Vista Sidebar offers some really cool potential for some awesome gadgets to be created.