Universal

Universal, best known for their Mr. Do! series, surely made original games. Their hardware isn't less original; it's actually one of the more unique and strangest ones in MAME. A lot of things in their boards were done differently from everyone else, and in non-obvious ways.
Even the style of the schematics drawings is unique and instantly recognizable - albeit not the easist to follow.

There is a long standing problem with the games Mr. Do's Castle and Do! Run Run, where the dip switch settings are not read by the game. The problem has been reported to happen even on real boards, so it is probably a timing issue (small deviations from the nominal rates of the components could throw the timing off enough to break it). For now, we'll fix it in MAME by just changing the CPU clock rate by a small amount. Kind of black magic, but it works.

Anyway, while studying the schematics I finally understood one thing that had eluded me until now: how the CPU communication happens on the real board. The two CPUs exchange bursts of 9 bytes, but there is only a single bidirectional buffer between them. How could it possibly work? Well, the Universal designers took advantage of the Z80 WAIT input. When the main CPU reads or writes the buffer, the WAIT line is asserted, so the CPU is put on hold. When the sub CPU reads of writes the buffer, the WAIT line of the main CPU is cleared, thus making it resume execution. So when the main CPU writes to the buffer, it will put the data there and pause until the sub CPU has read it. When it reads the buffer, it will pause until the sub CPU has written data to it, and resume execution (reading that new data).

Very cheap and effective. However, I wouldn't want to fly on a plane that uses this technique. :)