A new version of the experimental HelenOS operating system has been released. Version 0.4.0 features a simple user command line interface, FAT file system support, application loader and tracer, improved hardware support for ia64, ppc32 and sparc64, and many other improvements over the previous release. Get it from their download page.
The description of HelenOS is pretty self-explanatory:
The HelenOS project is an effort to develop a complete and usable modern operating system, yet offering room for experimenting and research. HelenOS uses its own microkernel written from scratch and supports SMP, multitasking and multithreading on both 32-bit and 64-bit, little-endian and big-endian processor architectures, among which are AMD64/EM64T (x86-64), ARM, IA-32, IA-64 (Itanium), 32-bit MIPS, 32-bit PowerPC and SPARC V9. Thanks to the relatively high number of supported architectures and suitable design, HelenOS is extremely-well portable.
Get it from their download page.