All of these songs are CC0 License ( http://creativecommons.org/publicdomain/zero/1.0/ ) and free to use for whatever you want.
These songs were made with a variety of virtual instruments. Some C-64 SID chip emulation, some Sega Genesis (Yamaha) FM-synth chip emulation, some NES Waveform synth imitations, some bitcrushed drum machine samples. Nothing pure or accurate, and not “true” chiptunes at all. I prefer the term Fakebit but it’s a less popular term.
I made some crazy music videos of some of these songs and will add more as I make them. Check them out on my YouTube Channel.
Credit is appreciated but not required.