BitVisor is a tiny hypervisor initially designed for mediating I/O access from a single guest OS. Its implementation is mature enough to run Windows and Linux, and can be used as a generic platform for various research and development projects.
A hardware supported hypervisor originally built for malware analysis. Features: Linux VM introspection, minimal detectability, small (~150KB), simple, and well documented. Can be used for other purposes. Support for Intel-VT & Windows coming soon.