BRiX: Advanced Computing Environment 
Fast, no-kernel, single address space, data-centric, preemptive multithreaded, persistent, secure, multi-user operating system, with Crush function-based programming safe-language. Some code available. [Open Source, Artistic]
http://brix-os.sourceforge.net/BRiX: SourceForge
Fast, single privilege, single address space, preemptive multithreaded, persistent, data-centric, secure, multiuser OS, with safe language, reconfigurable graphical interface.
http://sourceforge.net/projects/brix-os/
Choices Object-Oriented Operating System
Research OS coded in C++, architecture organized as frameworks of objects hierarchically classed by function, performance; customized by replacing subframeworks, objects; application interface is kernel objects exported via application/kernel protection layer; kernel and application objects examined via application browsers; runs on bare hardware on distributed and parallel computers. Virtual Choices (VChoices) also runs under Unix System V.
http://choices.cs.uiuc.edu/
SPIN
Dynamically extensible, Exokernel-based, provides many core services: scheduler, kernel threads, domains, event dispatcher, security mechanisms, primitive VM operations. Blurs distinction between kernels and applications, which traditionally live in user-level address spaces, separated from kernel resources and services by an expensive protection boundary. Lets applications specialize the kernel by dynamically linking new code into running systems.
http://www.cs.washington.edu/research/projects/spin/www/
Crush/BRiX: An Experimental Language/OS Pair
The usual variety of lively opinions, some of which are informed by study and quite insightful and useful in further understanding BRiX. [Slashdot]
http://developers.slashdot.org/article.pl?sid=02/08/18/2053259
