ForthOS was the basis for the author's own experimental software work; unlike many other Forth systems, this one as a real tool in day-to-day use as a part of a larger development. It is a fully standalone system, with a metacompiler used to generate new versions of ForthOS while running under ForthOS. Please read about its legal status; the short version is that ForthOS is in the public domain.
The current version is ForthOS 1.0. Please read about the installation here.
Actually, the last great copy of ForthOS development can be found here. There are bootable bits, and also a filesystem with all of the source.
Since viewing the source embedded within a block file seems to stymie many people, I've extracted the source tree into the src folder. The files are in pairs, with a ".txt" actual source, and a companion ".shd" which are the shadow comments.
For anyone interested, I wrote the script xfs.py to do the extraction.
There are some very old utilities which might help with the block files.
A private newsgroup server existed for a while to permit communication among interested ForthOS users; a read-only archive of it is here.