I suspect many of the companies are no longer in business any longer :~)
Let's start a list.
There was
ColorStudio early on. In 1990, there were two products for this task, one was Photoshop, one was ColorStudio. CS was more advanced (had CMYK before PS) but a difficult UI and workflow. At the time, it was like the Nikon vs. Canon wars today. I'm pretty happy I decided on Photoshop back then, but for awhile, I wasn't sure I picked the right horse.
There was the Kurger brothers who totally ripped of the PS UI with
X-Rez. We waited for a law suite but the company implode.
Before PS had layers, there was
College which did nothing more than create layers to send back to PS.
There was
Live Picture (who here paid the thousands to buy it?). Couple years later, it was bundled with scanners.
A product that never saw the light of day that I played with was
SpectraPrint Pro from Joe Mintz and company. Super high end, Scitex like app that never got released as far as I know.