LilyPond
A music engraving program where you type the notes as text and compile them into a score, much as TeX does for mathematics.
Han-Wen Nienhuys and Jan Nieuwenhuizen, two Dutch musicians who could also program, started it in 1996 because computer-set scores looked so much worse than hand-engraved ones. LilyPond became part of the GNU Project and is released under the GPL. Input looks like \relative { c'4 d e f }; output is a PDF whose spacing, beams and stems are computed to follow the engravers' craft.
The workflow is LaTeX's: no drag-and-drop, but full control, plain-text files that sit happily in version control, and results often handsomer than the graphical programs manage. The price is a learning curve that scares off most composers. The Frescobaldi editor lowers the barrier without removing it.