Microsoft .NET Gadgeteer is an open-source toolkit for building small electronic devices using the .NET Micro Framework and Visual Studio (including Express editions). Gadgeteer combines the advantages of object-oriented programming, solderless assembly of electronics with a kit of peripherals, and support for quick form-factor construction using computer-aided design.
Learn about .NET Gadgeteer-compatible hardware and see what people are building at http://netmf.com/gadgeteer.
If you're interested in building hardware for .NET Gadgeteer, you can find source code and interface specifications at http://gadgeteer.codeplex.com.