Let's summarize.
Between this thread and a couple of google/mylaps searches, I came up with 25 names associated with ITR cars. I'm not sure that all of these have turned a wheel yet.
Not surprisingly, the BMW 325 comes up the most often, with 10 names. It's the only car that was an easy crossover from another class.
Acura Integra Type-R: Goodnow
BMW 325: Andrews, Asinugo, Counts, Ferragonio, Jones, Lytle, Norris, Potts, Tisdale, Woodard
BMW Z3 2.8: Hughes, Sirota, Thompson
BMW M3 (E30): Hammitte
Honda S2000: Heth
Nissan 300ZX: Deems
Porsche 911SC: Smith
Porsche 944S2: Orr, Robertson, Wolverton
Porsche 968: Davis
Toyota Supra: Pawley
And then I have two names, but I'm not sure what car they drove (thanks, MyLaps):
Unknown: Mertz, Reighard
Not a thorough search. My guess is there have been more. But 25 cars (at least!) at this point, I'd consider that a huge success.