Rump kernel
The rump kernel is a NetBSD project that allows kernel drivers to be repurposed, specifically it allows applications to be linked into standalone unikernels that run on the Xen Project Hypervisor.
Essentially application code, the C library, kernel drivers for the network stack, and Xen Project PV (or HVM) drivers and some Mini-OS boot code are linked into a single executable that can be run directly on the hypervisor without an operating system. The resulting unikernel is small (often a few megabytes), fast to boot, and only contains the minimal dependencies necessary to run.
This is similar to MirageOS, but supports many existing Posix applications, rather than OCaml applications explicitly targeting MirageOS.
Resources
Documentation and source code is linked from rumpkernel.org. The main build tool for building Xen Project applications is rumprun.
There is an introduction in USENIX ;login: magazine October 2014.USENIX ;login: magazine October 2014.