Choral Public Domain Library

Contributors that are using software capable of converting proprietary formats into MusicXML are strongly encouraged to do so.

I'm also a MuseScore user (Linux/Ubuntu version) and the MusicXML format is apparently supported by over 50 notation applications, including Finale and Sibelius.

As it's and open source, virtually universal translator (Trekkies take note), it should be the default format for uploaders.