Modulation Synthesizer has 8 operators (ie oscillators) each of which can be mixed with other operators or can drive the frequency of other operators. It can create some nice, interesting sounds, but its not very good at many traditional synth sounds such as basses, or piercing lead sounds.
To use it you add the number of modules you want. For each module you then select whether it should be an ADSR envelope, mixer, sine wave, pulse wave or saw wave. Each module has 5 parameters (e.g. amplitude, frequency, variation, phase and delay for oscillators). Each of these can be driven by another module by selecting the number of that module to the left of the parameter. An indicator shows the amount of CPU being used.
ModSyn can be driven by MIDI. The shareware version is limited in that patch files cannot be saved and recordings can only be made at 11025Hz. Registration costs $30-$35.