Wikipedia
VSTa
VSTa (Valencia's Simple Tasker) was an operating system with a microkernel architecture, with all device drivers and file systems residing in userspace mode. It is mostly POSIX compliant, except when POSIX compatibility got in the way of extensibility and modularity. It was conceptually inspired by QNX and Plan 9. Written by Andy Valencia, and released under the GPL license. Currently the licensing for VSTa is Copyleft.
VSTa was originally written to run on i386 hardware, but was subsequently ported to several different platforms, e.g. 68030 based Amigas.
VSTa is no longer developed, but its ideas and source code are being used in the FMI/OS operating system, which continues the cause of VSTa.