I have a soft spot for AudioSim as it was the first software synth I ever used and was what inspired me to produce the first version of the Virtual Synth Page. Unlike all the other programs it runs under DOS. This means that there is no lag between turning the knob and you hearing the effect. There is also no lag between pressing keys on the (PC) keyboard and the sound coming out. Unfortunately it is unusable when running a MIDI keyboard through a Sound Blaster (due to the architecture of the Sound Blaster) though it can be used with a Midi keyboard through a Gravis. When being used with a MIDI keyboard it is possible to map the modulation wheel and note velocity to certain parameters.
AudioSim has a brilliant interface - looking just like an analog synth but on the screen. Everything is clear and you feel like you know where you are. It can read presets from disk, but they can only be saved in the registered version which also allows you to output to a wave file. It has 2 oscillators, 4 envelopes, 2 LFOs and a lovely arpeggiator, with a touch facility that allows the arpeggiator to run without retriggering the notes each time. There is also a little tracker type sequencer with bend facility. The filter is switchable between high pass, low pass and band pass modes.
Currently under development (though hidden away in the current version) are 'throat filters', which have multiple refraction and resonance points in the response curve of the filter. These act in a similar way to the throat and thus generate a 'voice' type sound. AudioSim is currently at version 1.91 and costs $30-40.