Haiku is a free clone of the BeOS.
It used to be called OpenBeOS.
Wikipedia explains:
What's Haiku?
What's BeOS?
Here's the official homepage.
Haiku R1 will be released "when it's ready".
In the meantime, if you're keen on trying it out, there are harddisk images and BeOS-compatible components available from the Build Factory.
There are full BFS images almost in lockstep with SVN revisions available from the schmid guy.
Most BeOS applications should work on Haiku.
You can get mine from here or from
the official BeBits app directory.
My contributions to Haiku, so far:
The Zip-O-Matic Tracker add-on
and a set of shell tools.
Any BeOS links, really.