Microsoft Research Singularity. A very different approach to designing an Operating System. Based on advances in modern programming languages, type systems, tools, static analysis, compilers and program verification. In fact, the majority of the team didn't even come from Microsoft Research's Operating System Division, they were language, tool, verification and compiler people.
The basic idea is that static typing and analysis can give much stronger guarantees about process isolation than the CPU's MMU can. And it can give these guarantees at compile time instead of runtime, which means that processes are much more lightweight. Lightweight enough, in fact, that every device driver, every file system, every subcomponent of an application, can run in its own, isolated, garbage collected, process with practically zero overhead. (A context switch is pretty much just flipping a bunch of pointers. There is no checking involved, since all of that has already been done at compile time.)
The ideas in Singularity are carried forward in some other operating systems out of MSR: Midori (which is rumored to be a commercial implementation of Singularity and a possible successor to Windows NT) and Helios (which takes the idea of having all device drivers, OS services and programs written in a hardware-independent language and applies it to a multi-kernel approach, where different parts of the OS run on different CPUs with possibly different ISAs).