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.
Infocom-style interactive fiction player
Frotz is a wildly popular and widely portable Z-machine. Ports are available for just about every machine out there.
Jainja is a JVM (Java Virtual Machine) written in Java
NedoPC SDK is a set of development cross-tools (compilers/emulators) to develop system and user software (and hardware) for some old home minicomputers and new "homebrew" hobbyist-friendly computers (including development of that kind of computers)