I am very much on the fence when it comes to including lilypond on this list.
As I understand it, lilypond — the program — just converts text-based sheet music into a visual format (I believe the home page calls it “engraved”) not unlike what LaTeX does … although I know very little about that either.
So the net effect of lilypond is this:
Text-based coding for sheet music converted into pdf format, without much of an interface or interaction.
That does drop it into a gray area though, where the benefit is in the output, and not the interface.
So is it a command-line application? Is it a true-blue text-only program?
I’m not going to fight through this one. I humbly submit that it does its job, interface or no interface, and if you need to draw some sort of line in the sand to reinforce an us-and-them mentality … then you’ve overlooked the real beauty of software like this — making life easier, and more beautiful.