I have created a PCB-version of my MidiJoy interface which lets you use your Atari 8-Bit or C64 as a chiptunes instrument (see http://www.phobotron.de/midijoy_en.html for more info). I'll put up the Gerber files online soon, so you can manufacture your own. However, since minimum order for this test run was 10 pieces, I have a few spare sets (Teensy microcontroller not included) which I would give away for material costs. PM me if you are interested.
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I have added a very simple sample code on my website which shows the basic structure of the MidiJoy software. It should be easy to translate this code to computers/consoles other than the Atari or C64. At the same time, the interface could stay as it is, so hopefully the devices that can be used as instruments will increase in the future 
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BTW, there are still a few PCBs left, in case someone is still interested...
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Good news: After the Atari 800 and the C64, MidiJoy now also works (rudimentary) with the VCS 2600 console. Source code as well as binary for usage with a Harmony cart can be found on my website.
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And more good new: The MidiJoy interface can now also be used to play pre-recorded digi-samples on the C64, Atari XL/XE as well as the VCS 2600. I still have a few prototypes here which I'd give away for material costs (comes without Teensy controller and cables).
http://www.phobotron.de/midijoy_en.html#Samples
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Here's a YouTube video demonstrating the digi-samples on an Atari XL - wirks likewise on a C64. The last sample demonstrates the manipulation of playback speed, all controllable from your Midi keyboard/seqencer:
http://youtu.be/qOZ4FaymSZY
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I have added an 8 minute video on YouTube as well as on the project website that shows the playback of 5 different tracks on the C64 using MidiJoy, enjoy:
https://www.youtube.com/watch?v=nYBQy-TMExs