This Java-program plays MusicXML files and can save them as Standard MIDI Files. It is available both as a Java application and as a Java applet and is free software (GPL). Not maintained any more. Use the Zong! Player instead.
MusicXML is a file format developed by Recordare that can be used to exchange files between music notation software like Sibelius, Finale or Igor Engraver.
Xenoage Player is a program written in Java that plays MusicXML files and allows to save them as Standard MIDI files. It is available as a standalone application and an applet for webpages. From version 0.4 on it supports also MusicXML 2.0 files.