Dodo Bird has released Super Eel v2.0, a free VST effect plug-in for Windows that functions as an Audio to MIDI processor as well as a fully fledged synth and effect unit. The full source code is also available.

Super-Eel does 4 different things:

1. Audio In » Synthesized sound Out
   Like Super Eel 1.0, Kantos, Alien Solo, etc.
2. Audio In » MIDI Out
   To play another synth.
3. MIDI In » Synthesized sound Out
   Just like a normal VSTi, read the manual to see how to use this feature.
4. Audio In » Processed audio Out
   Super Eel 2.0 contains pitch following filters, delay and distortion.