The HelenOS project is an effort to develop a portable operating system with some experimenting and research in mind. HelenOS supports SMP, multitasking, and multithreading on 32-bit and 64-bit, little-endian and big-endian processor architectures, among which are AMD64/EM64T (x86-64), IA-32, IA-64 (Itanium), 32-bit MIPS, 32-bit PowerPC and SPARC V9. HelenOS has also been recently ported to Xen 3.0. The extent to which these architectures are supported differs from architecture to architecture. Some run on real hardware, and some run only in a simulator.
One of the unique things about this OS is its IPC mechanism and the way it handles connections via set of lightweight worker pseudothreads.
The project has been going for some time already, but it is still not listed in OS Dev Wiki. You can find its homepage at http://www.helenos.eu.
From the project site you will learn that the microkernel is fairly complete and even uses some advanced techniques (e.g. an implementation of SMP-optimized slab allocator). Nevertheless, the system lacks some nice-to-have things on the userspace/services front (e.g. filesystem) that prevent it from being more usable. If you are the kind of a developer that likes to delve deep into system programming, HelenOS might be the right project for you.
You can contact HelenOS developers via their mailing list or write an email to jermar at helenos dot NO_SPAM eu.