Haiku,
http://haiku-os.org( formerly known as OpenBeOS )
The open-source (mostly MIT) effort to re-implement BeOS R5 as Haiku R1 can now be booted directly from your hard drive!
Currently, Haiku is only command-line utils as the app_server is still in intense development. While Haiku itself is only CLI, many other programs, libraries, etc... are ready and can be already used in BeOS R5 ( eg, preference apps, translation kit, input kit, mail daemon replacment, to name a few).
Regardless it's very nice to have something bootable to play with.
Haiku's source tree can be downloaded, compiled, and packaged into a BFS image file, which can then be extracted onto a BFS partition.
Here is a pre-made image file, simple expand, mount the image, and copy to a BFS partition.
http://www.schmidp.com/public/misc/haiku.image.zipany chance of getting updating the OS listing on the main page to reflect the new name and website change?
