LilyPond
LilyPond is a free, open-source music engraving program that produces high-quality sheet music from text-based input files. It uses a markup language similar to LaTeX to describe musical notation, which is then compiled into beautifully formatted PDF or other output formats. It is widely used by composers, arrangers, and music publishers for creating professional-looking scores.
Developers should learn LilyPond when working on projects involving music notation, such as digital music libraries, educational apps, or automated score generation tools. It is particularly useful for batch processing large sets of scores, integrating with other software via its command-line interface or APIs, and ensuring typographically precise and aesthetically pleasing output that is difficult to achieve with graphical editors.