The latest release of HelenOS makes for interesting comparison with the HURD and MINIX. Like them it is minimalist, multi-process rather than monolithic, and has UNIX-like commands. HelenOS is written in c and is cross-platform.
However it is a newer design which came out of assorted PhD theses and the like, which the developers hope will give it mileage down the line.
Distrowatch have an interview with Jakub Jermar, one of the developers.
Article in LXF any time soon, or have I already missed it?