Microsoft Research has released the Singularity Research Development Kit (RDK) and it is now available including source code, build tools, test suites and other materials. What is Singularity? From the project site at CodePlex where you can get the RDK:
"Singularity is a research project focused on the construction of dependable systems through innovation in the areas of systems, languages, and tools. We are building a research operating system prototype (called Singularity), extending programming languages, and developing new techniques and tools for specifying and verifying program behavior.
Advances in languages, compilers, and tools open the possibility of significantly improving software. For example, Singularity uses type-safe languages and an abstract instruction set to enable what we call Software Isolated Processes (SIPs). SIPs provide the strong isolation guarantees of OS processes (isolated object space, separate GCs, separate runtimes) without the overhead of hardware-enforced protection domains. In the current Singularity prototype SIPs are extremely cheap; they run in ring 0 in the kernel’s address space."
One of the interesting things about Singularity project is that it is written in managed code - C#. This makes two such OS released recently. I wrote about COSMOS recently. This is an interesting time for people interested in learning how an operating system really works. There are more source code options then I can ever remember.