Stanimir Stoyanov has developed a Sidebar Styler which among other things allows Windows Presentation Foundation (WPF) to be run in a Sidebar gadget. What does this mean? Well, WPF is a very rich graphics subsystem with which designers can create stunning looking applications. Up until now, gadgets were basic HTML files which were kind of clunky to build with some awkward scripting thrown in. Now you can exploit the .NET 3.0 framework from within your gadget - which is very nice indeed!
Stanimir has also released a tutorial on how to develop your own WPF gadget.