Butterfield Overland was already mentioned.
There were also Northwestern Express, Holladay Overland, Barlow Sanderson, Rockwell, Deseret, Risher and Hall, American Express, Blackhills Stage and Express, California Stage Company, and maybe a couple of hundred that only had a stage or two (or three) and followed a short route. W.T. & Co. Express bought out a bunch and forced a large number of others out of business.