How can I write gadgets for the Windows 7 desktop using C# and Visual Studio 2008? I'm looking for tutorials and resources on that topic.
Try Authoring Sidebar Gadgets in C#.
C# is not the only option. This detailed tutorial explains how to develop gadgets without C#. (Orginally gadgets were authored in a mix of XML, HTML, CSS, and some IE scripting language.)
How to get started with Windows 7 gadgets is a related Stack Overflow question that may also provide additional information.