Little Sound DJ
Used by 3 peoplelittlesounddj.com ↗

“For Game Boy stuff, I use a piece of software called Little Sound DJ, a.k.a.”
Paul Weinstein Interview, Chiptune artist · via Uses This 
“For hardware chiptune on Game Boys I use a program called Little Sound DJ (LSDJ for short), which is a synthesizer and sequencer built into a Game Boy cartridge.”
James Primate Interview, Musician, game developer, artist (Rain World) · via Uses This