VSTa http://www.zendo.com/vsta/
Not only does VSTa have a deliciously consistent and simple
architecture, but the code is some of the best I have ever seen in
public-domain software. I am working on a port of xkernel so it can
have some networking other than ka9q.
Eros http://www.cis.upenn.edu/~eros/
This system borrowed heavily from the concepts of KeyKOS. It will
thankfully be released under the MIT public license rather than the GPL.
Exokernel http://www.pdos.lcs.mit.edu/exo.html
Some impressive performance numbers, but the code needs serious
clean-up. The most interesting feature to me is their DPF network
abstraction.
Ben