OSv is an operating system for virtual machines in the cloud. It can run existing Linux code, including a JVM, but is much smaller and simpler than Linux, and has been designed from scratch to focus on running efficiently on virtual machines.
Utopia (formerly, Mobile Code OS) is an operating system for mobile code run in an environment of fine-grained, customizable virtualization.
A Message Transfer Agent for Unix systems.
A framework to implement custom command line interfaces (CLI).