The Foo YC20 is an open source implementation of the Yamaha YC-20 combo organ from 1969. Processing for the organ is based on original schematics and measurements from a working specimen. This instrument simulates the circutry as a whole to realisticly reproduce the features and flaws of the real deal.
You can play the instrument using your keyboard, a mouse, or with an attached MIDI keyboard. Google Chrome 43+ recommended.
DSP for this organ is written in Faust, which is a functional programming language dedicated for writing DSP.
This instrument is also available as a VST plugin for Windows and OS X, and as a standalone client and LV2 plugin for Linux. Source and downloads are available on github.
More info is available on codeforcode.com