I started working with computers on HP-3000 minicomputers. Yeah, what fun it was to code in SPL! The 3000 had a stack-based architecture, reminiscent of the Burroughs mainframes of the sixties and seventies.
According to the Wikipedia, versions of the 3000 are still in use. And I see Bob Green of Qedit fame is still at it.
You don't run into folk that even know what the HP-3000 was very often. I still have a copy of Eugene Volokh's "Burn Before Reading". The 3K had a really weird way of organizing files that was completely different from the Unix way of doing things. There is a company out there that has written a port for the RISC-based version of MPE that will run in VMWare. It's pretty neat to boot up the VM and see a ":" prompt. Until fairly recently there was a system out there that had a telnet server running on it where you could login and play Warp. I'd like to find an old Model 70 somewhere to heat my house during the winter. (grin)
Regarding Bob Green, it's amazing they can still keep that going since MPE has been dead for more than a decade now. They certainly have a niche. Hope it's keeping the lights on.