LilyPond

From the LilyPond website:

Music Notation for Everyone

LilyPond is a music engraving system designed to produce sheet music of the highest quality. It brings the aesthetics of traditional hand-engraved music to computer printouts. Because it is controlled entirely via text files, LilyPond is highly accessible and offers outstanding flexibility.

Key Features

Beautiful Classical Engraving

Produces layouts inspired by traditional hand-engravers, with optimal spacing, beam slope algorithms, and elegant font design.

Accessibility-Friendly Workflow

Being a code-based text system, LilyPond is exceptionally accessible for visually impaired composers and blind musicians using screen readers.

Text-Based Input

Write scores in simple text files (.ly). No graphical user interface required, enabling developers to code music with high speed and precision.

Free and Open Source

Completely free of charge, with an active, helpful open-source community developing tools and extensions.

Versatile Output & MIDI Support

Compiles instantly to PDF sheet music, SVG graphics, and fully customizable MIDI audio files.

LMS and Command-Line Integration

Integrate LilyPond into websites, LaTeX publications, web applications, and automated grading pipelines.

Example LilyPond Code

\relative c' {
  \clef treble
  \key c \major
  \time 4/4
  c4 d e f | g a b c |
}