Image via WikipediaMusopen is a free source of public domain music, both recordings and sheet music. They are also creating an open source music theory textbook. I would love to see an open source music appreciation course using these materials. This site combined with other free resources like the Wikipedia Sound List would provide a lot of material for such a course
"Musopen is an online music library of copyright free (public domain) music. We want to give the world access to music without the legal hassles so common today. There is a great deal of music that has expired copyrights, but almost no recordings of this music is in the public domain. We aim to record or obtain recordings that have no copyrights so that our visitors may listen, re-use, or in any way enjoy music. Put simply, our mission is to set music free."
The music is very well organized and down-loadable. Also you can embed the music in a web page so if one was to create a music history textbook in a wiki, one could easily use this. I love the example I chose below. "Entrance of the Gods into Valhalla" was the processional at my wedding and the Marine Band does some interesting things with it that really allow you to hear the textures and layers in this piece: