Let's not forget these Alternate OS for old systems.

* Dave Braun at Intel wrote UZI (a unix v7 ish system for Z80 processors, and public domain) - one of the gnu project mistakes was not building off this but using Mach
http://www.dougbraun.com/uzi.html
* Steve Hosgood at UW Swansea wrote OMU - a 6809 and later 68000 based platform with a unixlike api
http://tallyho.bc.nu/~steve/
* Jawaid Bazyar of EGO systems wrote GNO/ME a unix like Multitasking Environment for the APPLE IIgs 65816 computers.
http://www.gno.org
* commodore/64 - Daniel Dallmann, lunix
http://lng.sourceforge.net/
* msx - Adriano C. R. da Cunha, uzix
http://uzix.sourceforge.net/

Also

* C64 GEOS

* Contiki operating system and desktop environment for the Commodore 64, with ports to a bunch of other platforms such as the 8-bit Nintendo Entertainment System, the VIC-20, 8-bit Ataris, Atari Jaguar, the Tandy CoCo, and the Apple ][ under development. The Contiki system includes the following: a multi-tasking kernel, a windowing system and themeable GUI toolkit, a screen saver, a TCP/IP stack, a personal web server, and a web browser. The Contiki web browser, which is likely to be the world's smallest browser given its extremely small memory footprint, is the world's first true web browser for an 8-bit system and probably makes the 21 years old Commodore 64 the oldest system ever to run a real web browser! All of the above programs are contained in a single, fully self-contained, 42 kilobytes large binary. The entire Contiki system with all programs running simultaneously is comfortable in 64 kilobytes of memory.

FYI: I'm a OS and Retro System Freak OK...
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