PowerPC is more of a niche product now that Apple doesn't use it any more, but it is still used in IBM servers and in high reliability military/aerospace applications.
Well, not to mention all three of today's major game consoles (PS3, X360 and Wii) use the PowerPC architecture. I think the majority of the computer boxes used in cars also use PowerPC, as well as, as you've mentioned, aerospace, which includes jet fighters and such.
SPARC is still used by Oracle and some supercomputer vendors, but was somewhat marginalized by Sun's decline and the rise of cheap x86 servers (SPARC is interesting in that there are Open Source implementations of the ISA).
In fact, SPARC is the ISA of choice for Japan's K computer, today's 2nd fastest supercomputer. Not bad, but Fujitsu (and others) need to keep moving this ISA forward.
MIPS has also been quite popular in the video game console market during the past few generations. I think the Playstation 1 and 2 as well as the Nintendo 64 all use MIPS.
Had Don Estridge and his team chosen, say.. Motorola's CPU at the time, I suspect that the 68k ISA would have become the x86 of today
Very good point, bjm. And Intel wouldn't be the giant it is today. Imagine if Motorola is what Intel is today.
Personally, if the world is going to do everything again I would've chosen SPARC. The spec is open and, as I understand it, is free for anyone to use. That would at least make sure that the industry can continue without fear of one company trying to rake it all in for itself. But I wouldn't have liked it to turn out like ARM or MIPS, where the innovator company would design the cores for licensing to other companies. Instead, I would have liked the SPARC standards body to just put up the spec out there, with Microsoft supporting the OS perhaps, and companies can just create their own core designs as well as system architectures (such as something like Hypertransport) to support the dominant OSes.
The only way not to grow old is to die young.