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.
Alternatively, run it on an emulated PC with the files found 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 a source viewer. The source viewer lets you flip between the source screen and its shadow comments (if any). You will find the source clumps a bit; remember that the source was accessed and edited in individual 80x25 screens.
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.