A website for playing sequenced music (video games, chiptunes, and MIDI files). macOS lost native MIDI playback a long time ago. And I like to listen "interactively," adjusting tempo, soloing instruments, stuff like that.
I have gone deep on a few problems, like converting Nintendo 64 to MIDI+soundfonts (there are tools for this, but nothing turn-key) or enabling support for Farbrausch V2 (a powerful but tiny synth engine). There's really no point other than to solve puzzles with a very salient musical reward at the end. https://github.com/mmontag/chip-player-js