We know that alternative OSs exist for many older platforms, as the enthusiasts try to continue to get more from their aging hardware.
The c64 GUI OSs are well known and very impressive for an 8-bit platform, and many different kernels and OSs have been released for the Atari ST to try and get away from the limitations of it's horrible default GEM desktop.
But what about other systems? 8-bit systems shipped with a commandline only OS as their main interface, but these days some mad people are writing some amazing things for them still.
Have a look as SymbOS. This is a fully multitasking preemtive OS for the Z80 based 8-bit computers in the Amstrad and MSX ranges. http://en.wikipedia.org/wiki/SymbOS
What alternative retro OSs do you all know about, and have you tried any of them?