RetroTom Novelli's original "Retro" was a FORTH-based OS and a prototype low-level infrastructure for the TUNES project.
RetroForth is now developed and maintained by Charles Childers. It still aims to be simple and fun, but the emphasis is on "playing nicely" with popular OSes. It currently supports Linux, FreeBSD, Windows, BeOS, and DOS.
While there is little interest in a Forth-OS for x86 hardware, Charles still enjoys playing with the Forth-OS idea. Recently a lot of development has gone into it, and it's now suitable for embedded x86 work. Others are seeking to use this as a basis for a more powerful operating system, but little progress has been made on that front.
(Author's note: I abandoned Retro because FORTH did not live up to my original expectations. A FORTH compiler is just as complicated as any other compiler, but the language is difficult to use; other languages provide more "bang for the buck". It was also overwhelming to write a complete OS which wasn't usable until the whole thing was done.. this "revolutionary" approach was impractical, so I've dropped it in favor of making evolutionary changes to Linux. For more postmortem details, see my Conclusions article and the Addendum. -- T.N.)