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 .
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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:
goattracker
This piece of software allows you to make songs by emulating a Commodore 64 sound chip. Great stuff, and it's free to download!