The purpose of MusicXML, developed by Recordare and adopted by many vendors is a royalty-free XML schema for distributing and sharing interactive sheet music with a wide variety of musical applications. MusixXML is implemented in particular most scorewriting programs like the commercial Finale and Sibellius or the free MuseScore
“The MusicXML format represents common Western musical notation from the 17th century onwards. It lets you distribute interactive sheet music online, and to use sheet music files with a wide variety of musical applications. The MusicXML format is open for use by anyone under a royalty-free license, and is supported by over 150 applications.” (Recordare.com)
The advantage of MusicXML with respect to MIDI, is that this language was specifically targeted for notation, e.g. it can include text.
All MusicXML versions are upwards compatible
The Notation Interchange File Format (NIFF) is an alternative. However, according to Wikipedia, , retrieved 16:29, 12 September 2011 (CEST) “NIFF is now considered obsolete mainly due to the MusicXML format. As of February 2006 the NIFF project web site has been closed.”
See MusicXML software list for a complete list.