Cynthia is a free Windows VSTi/VST inspired by the classic EMS Synthi A.
Specifications (like Synthi A):

3 oscillators with controls for shape and level of the waveforms

OSC1: Range from 0.6 Hz to 16 kHz, sine and triangle
OSC2: Range from 0.6 Hz to 16 kHz, square and triangle
OSC3: Range from 0.015 Hz to 500 Hz, square and [...]

A great video introduction to SynthEdit - the Windows virtual synth creation software.
Given that SynthEdit is shareware and Macs are Intel now, why hasn’t anyone done a Mac SynthEdit?
Build A Synthesizer With SynthEdit, Part II

Build A Synthesizer With SynthEdit, Part II