Open Source

AtomsNet
Modular object-oriented application to index files across networks. Has plug-in type data retrieval system, webserver, peer-to-peer connections for network-wide information gathering, retrieval; has features of OSs, database management systems, distributed networking. Open Source, GPL.
http://atoms.sourceforge.net/

E1
A distributed operating system based on the concepts of object replication, component model support and persistence. It consists of a L4 microkernel and a set of distributed objects acting at the user level. [Open source]
http://www.e1os.org/

ILIOS
InterLink Internet Operating System, single tasking, uses interrupts for processing, for routers only and maximum throughput on i386+ systems; but in tests it barely beat NetBSD on same hardware, so author stopped work. Has fast IPv4 stack, based on XeOS codebase. Open source, BSD.
http://www.rink.nu/trac/ilios/

Mungi
Orthogonally persistent, capability-based secure SASOS using L4 2nd generation microkernel. Goals: Prove SASOSs can run on normal hardware, be as secure as normal OSs; can be as efficient as, and are faster than, normal OSs in some important uses; can be pure SASOSs with all data (even system data) in one address space and no other IPC mechanism is supported by OS. POSIX emulating. [Open Source, GPL]
http://www.cse.unsw.edu.au/~disy/Mungi/

Quarks
Simple, yet efficient distributed shared memory (DSM) system; a user-level library plus header files supports DSM on groups of Unix workstations. Runs on 4.3BSD/M68k, HP-UX/PA-RISC, IRIX 5.2/MIPS, SunOS 4.1/SPARC. [Open Source, public domain]
http://www.cs.utah.edu/flux/quarks.html

x-kernel
Object-based framework to implement network protocols; defines an interface that protocols use to invoke operations on one another (i.e., to send messages to and receive messages from adjacent protocols) and a set of libraries to manipulate messages, participant addresses, events, associative memory tables (maps), threads.
http://www.cs.arizona.edu/xkernel/