At composer Jon Magnussen’s site you can download clips of his (incredible) pieces as well as the sheet music in PDF format. How’s that for open source?
Interesting: When you visit his site, which genre of music do you click on first?
Mine is "Chamber". Yours?
This kind of sharing is unusual in popular/commercial genres of music, but there's been a flourishing "open source" online community among traditional musicians for more than a decade now. For traditional Irish music, for example, there are lots of web sites where you can download transcriptions of tunes, and trad musicians have even developed a code called "abc" that allows people to share text-only transcriptions of tunes via e-mail. You copy the abc code into a software reader, and it converts it into standard musical notation that you can print out or play on your computer. Popular abc readers are ABC2Win for Windows and Barfly for the Mac.
Some modern composers of traditional-style tunes eschew copyright and put their music in the public domain. Most of us who do this figure we wouldn't make much money off our compositions anyway, and our real reward is hearing our tunes played and recorded by other musicians.