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 hypervisor.
Essentially application code, the C library, kernel drivers for the network stack, and Xen PV (or HVM) drivers and some Mini-OS boot code are linked into a single executable that can be run directly under Xen 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 targeting Mirage.
There is an introduction in USENIX ;login: magazine October 2014..