Probably all not *really* rare, but rather odd and not readily available:
Mattel Aquairus - See these from time to time. Should've got the tape drive
while I could from Salvation Army.
Bell & Howell Apple II OEM - I guess these are somewhat rare. My school had
a whole bunch of them, and threw almost all of them away. Probably the fate
of most Apple IIs.
Data General Nova 3 - Probably not that rare, but I don't see many.
Apple IIc Plus - Only made for a year or so. Again, not ultra-rare, but not
often seen around.
Magnevox Odyssey 1 - Not really a computer (It was analog!), but farily
uncommon, and predecessor to the really-a-computer Odyssey 2.
Sharp plotting calculator - Forgot the model number, it's in the archives
somewhere. A battery operated desktop calculator with 4-color pen plotter!
This is the only calculator I've seen with a built-in plotter.
Quadex Q500 - Desktop publishing minicomputer. Can't even find info online
about this thing. I guess it was one of the first minicomputer-based DTP
systems, even has a special desk the CPU and 8" Floppy/winchester disk fits
into.
Creative Labs 3DBlaster - A 3DO game system on an ISA card. Not many were
made, and 3DO didn't live long.
IBM EduCase Model 25 - A big, chunky 486 multimedia version of the classic
PS/2 Model 25 all-in-one computer. Only sold to educational markets, not
many were bought (They were EXPENSIVE)