After looking at them for a while, trying to condense them down to just a few choices, these ones look the most interesting:
Vex Chords
The Vex Chords code examples look easy to write, the output looks great, and it has many configuration options.
Fretboards
For frontend-generated JS-based fretboards, Fretboard looks good.
Fretboardgtr
For SVG fretboards, the Fretboardgtr Python library looks good.
These other ones aren’t quite what I’m looking for, but they look like they would be useful for certain tasks:
Lilypond
For sheet music, Lilypond produces great output.
ChordPro
ChordPro looks great for text-based lyrics with chords.