Cosmos looks like a project that could be fun to play with. It's goal is to allow you to build your own operating system in .NET!
The magic happens by compiling .NET CIL bytecode to x86 assembly.
The core is written in C#, but you should be able to use any .NET language to add your own extensions.
You can deploy your build as an ISO or PXE, or run it under a number of emulators like QEMU, VMWare and VirtualPC.
The project is still in the early stages, providing just console interaction. Future releases will add PCI enumeration, Ethernet, TCP/IP, extended file system support and maybe even GUIs.
UPDATE: There appear to be a couple of competitors, the open source SharpOS (very, very early stages) and Microsoft Singularity (available only to academic institutions).