Operating Systems based on Micro-Kernel seems to return fashionable, and some experimental OS releases demonstrate it: the National ICT of Australia, infact, has released a Darwin porting for L4. This is for being able to study the behaviour of that class of o.s..
L4 is, collectively, a family of related computer programs. They are microkernels that are becoming well known in the computer industry for their excellent performance and small footprint...The source? As usual...