- National 32032 CPUs. Basically a VAX-on-a-chip, they were relatively slow and pretty buggy. (They were one of the processors we considered using in the Atari ST, but the 68000 won out, thank goodness).
- INMOS Transputer. One of the early "just apply +5V and ground and happy computing!" systems with zero support chips needed, quite easy to put into a grid of compute elements. Very interesting nibble-based instruction set, worth studying. Unfortunately the high-level language INMOS was pushing (Occam) was really strange, and Transputer performance was never really great. There were also some cool microcode bugs (you could lock a Transputer up for minutes by executing a bit-shift instruction with a very large count).
- RCA 1802. Another early microprocessor. Not really a failure, but not a huge success, except for its radiation-hardened variants, which have flown on many space missions.
- Just about any floating-point coprocessor chip. Ugh. Thank goodness the early Pentiums stopped that madness [1].
[1] I heard that Intel wanted to charge for doing floating-point operations on Pentiums. The idea was that you'd pay to have an "enable" fuse blown, which would purchase (say) 100M operations before a disable fuse got blown. There'd be something like 64 fuses, and the last one would enable floating ops forever. The rationale was that the only people who really cared about floating point were folks running spreadsheets . . . and those people obviously had money and would pay for performance! People hated this idea; then 3D gaming started to be popular (with Quake, et al) and suddenly flops were in general use and it would have been a really awful marketing move by Intel.