X11 client and server. DOS. Windows 3.1. All with preemptive multitasking.
I got a copy of DESQview (and later DESQview X) from a friend of mine in the mid-90's (I was a poor teenager at the time so I didn't object to sternly to copying-that-floppy). I adored both platforms. What makes them even better is that the real kernel was the QEMM memory manager. That's what transformed DOS into a multitasking workhorse.
DESQview/X was better than Windows 3.11, and in my view, at least as good as Windows 95 in terms of capabilities.
I'd love to see FreeDOS continued to the point where it has a native shell like DESQview/X. Legacy 32-bit DOS extender computing just makes me unreasonably happy.
I had DESQview (without the X) running on DR DOS 6 in the early '90s. I had built a generic clone computer (12-MHz 286, 2 MB RAM, 120 MB disk...and a 14.4-kbps modem) to run my BBS so I could free up my Apple IIe to use without knocking the BBS offline. When I added on DESQview, I could run some small programs in the foreground while the BBS kept going in the background. It was rock-solid when doing this, too...never a hint of anything remotely similar to a BSOD.