For many years the "big four" were Nikon, Minolta, Canon and Pentax, in that order, with Olympus and Mamiya right at their heels. Nikon pretty much held the lead throughout the film era with the F series being the best cameras produced. Where the real shakedown in the market began was with the development of auto focus technology (originally developed by Honeywell).
Nikon managed to stay in the lead but just barely with Canon coming on strong. Minolta and Pentax were pretty much out of the running. Canon made a major breakthrough that threatened Nikon's position with the introduction of the EOS-1 in 1989.
This was followed with the D30, 1D and IS lenses in rapid succession, and Canon's position was solid. It's only in the last couple of years that Nikon has managed to catch up. Pentax now holds the number three position (barely) and is developing better cameras as time progresses. They still need a full frame, though. We all know what happened to Minolta...
Olympus has managed to create a niche market with the 4/3 format, but they will never be a credible threat to Canon or Nikon unless they produce a full frame (35mm size) digital, with the lenses to go with it. Olympus was one of the pioneers of small format cameras—they produced a series of half-frame 35mm cameras in the 1960's and 70's—and seems to be continuing this way.