AFAIK the FF is unique. There is the Vexter version of the pedal. There are always used ones around. Its a challenging pedal to use-every tweak of the knobs reveals different sounds, its not a plug it in and go sort of pedal.
Zoom had a pedal called the ultra fuzz that could do some of the theremin fx, but wasn't capable of all the craziness of the FF.
I had a Fuzz Factory for a while. It only did one thing that I liked and it didn't seem to agree with active pickups. If you go to this site and scroll down to the PWM, there is a schematic that does something similar to the thing I liked about the FF.