The earliest bicycle disc brakes I'm aware of are the '70s Shimano brakes mentioned earlier, though I strongly doubt they were the first commercially available disc brakes for bicycles. You could buy them on bikes from Kmart, so there's no doubt they pretty much sucked. Despite being made by one of the big name manufacturers, they failed to catch on at that time.
The next widely sold bicycle discs were made by Sachs and Mountain Cycle in the late '80s. Sachs made small diameter rotor hydraulic discs that were similar to fancy moped items from the time. The most noteworthy thing about Sachs discs were that they brought the 44mm 6-bolt rotor mounting pattern that has become the predominant standard since then.
Mountain Cycle made cable-pulled hydraulic master/slave calipers and full-floating hard anodized aluminum rotors under the name "Pro-Stop". Those were seriously hyped as the Next Big Thing by publications like Mountain Bike Action. I have a set of Mountain Cycle Pro-Stop brakes with 9" rotors. They work fairly well, but they jingle a lot when rolling and they make a horrible honking racket in wet conditions.
Magura, Hope, Hayes, Formula, and Avid were part of the next wave of commercial development in the 1990s that left us more or less where we are now.