386BSD takes us back to the days when you could count every file in your Unix distribution and more importantly, read and understand all of your OS source code. 386BSD is also the missing link between BSD and Linux. One can find fragments of Linus Torvalds's math emulation code in the source code of 386BSD. To quote Linus: "If 386BSD had been available when I started on Linux, Linux would probably never had happened."
Though it was designed for Intel 80386 microprocessors, there's already instructions for launching it on the hosted hardware virtualization service Qemu.
ReactOS is free. You can boot your desktop or laptop from it. It looks like Windows (a 10-year-old version, anyway), so you already know how to use it. And it'll run some Windows and DOS applications, maybe including DOS games that regular 64-bit Windows can no longer touch. These videos even show ReactOS running Elder Scrolls: Skyrim and Doom 3.
Well now, three decades later, Kevin Savetz of the excellent ANTIC podcast has dropped a bomb on the retrogaming community: there was a real Star Raiders II under construction for a long time, but it disappeared as Atari imploded. Kevin tracked down the author, Aric Wilmunder, and convinced him to release it after all these years. You can download the game for the emulator of your choice, and read the manual and backstory on the Internet Archive.