MusicXML
An XML format for sheet music that lets one notation program open another's work without losing beams, accidentals and voices.
Michael Good and his company Recordare released version 1.0 in 2004. Until then the MIDI file was the only common language, and it describes when notes sound, not how they're written: F sharp and G flat are the same key, and a triplet becomes three oddly placed notes. MusicXML stores the notation itself in XML — spelling, slurs, lyrics and page breaks included.
It became the de facto standard, supported by Finale, Sibelius, MuseScore and Dorico. MakeMusic bought Recordare in 2011, and in 2015 the spec moved to a W3C community group. Round trips are rarely lossless — every program has its own opinions about layout — but they beat re-entering an orchestral score by hand.