The L4 ecosystem is interesting and there's a lot of activity going on. If you're just interesting in something 'not OSX/Windows/Linux', there's the IBM Mainfr

The L4 ecosystem is interesting and there's a lot of activity going on. If you're just interesting in something 'not OSX/Windows/Linux', there's the IBM Mainframe world (you can use the Hercules emulator to run the software) or the DEC nee HP VMS world (there's a free hobby license). Way out, you can run Symbolics Genera in a VM and see how the world would have been if Lisp had won.