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 MusicXML Player is a program written in Java that plays MusicXML files and allows to save them as Standard MIDI Files. It is free for both personal and commercial use and is currently available as a standalone application and an applet for webpages. At the moment the Xenoage MusicXML Player is still in beta stage.
This program runs on Windows, Linux, MacOS and all other operating systems that support Java. You need at least the JRE 1.4.2.