NES meets the C64 SID
Here's a DIY hacked Cynthcart, which is connected to the NES, making this a synthesizer with the power of both the C64 SID and the NES audio chip.
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cajoNES, the NES drum synth / sequencer
Here's a free NES noise generator for you! Check out also the other NES audio tools by Neil Baldwin .
DIY NES synth
Here's a great DIY build: a small keyboard imitates 4 NES controllers that are connected to the original NES running a NESK-1 synthesizer cartridge . Check out also this demo of the NESK-1: