Arppe2600va is a free VST Synth Instrument which has a multitude of Frequency Modulation options which enable it to create some truly awesome sound effects. PWM, Sample & Hold, virtual Spring Reverb, Ring Modulation, and a dedicated Noise Generator are all there! Also, there is Oscillator Sync and Filter Color (to adjust between LowPass and HiPass filter types).
You can have up to 12 patch routings with a very user-friendly (and visible!) drop-down menu interface, instead of messy virtual patch cables cluttering up the screen. All controls are mapped to MIDI CC#’s, and you can override the default settings by using the MIDI Learn feature. Use as a simple stand-alone synth (.exe file included), or copy the DLL into your favorite DAW or sequencer’s plugin folder.