MIDI2600
MIDI-to-Joystick / MIDI-to-Keypad Adapter for Atari 2600
http://highlyliquid.com/kits/midi2600/
The Synthcart is an Atari 2600 cartridge that turns your 2600 into a pop music sensation, letting you play music using a pair of Atari's keyboard controllers and can also be controller with the 2600 MIDI kit.
http://www.qotile.net/synth.html
MIDIbox TIA - Atari 2600 Synth
http://midibox.org/forums/topic/18032-midibox-tia-atari-2600-synth/
Schematic for MBHP Platform, Thorsten style:
http://midibox.org/forums/index.php?app=core&module=attach§ion=attach&attach_id=8838