I'm wanting to log this information for easy access later, and I figured I might as well do it here so you can know about it, too.
Three of my children are studying piano, and we are often wanting to listen to a recording of a particular piece they are playing. While their piano teacher has an extensive collection of Classical CD's, we do not, so we are thrilled to have found www.musopen.com.
For Classical, Romantic, and Baroque-era music - as well as the 20th century composers - visit Musopen. You can browse the site by instrument, composer, or performer. You'll find many free music MP3s that will work on computers and portable music players; you can even download the sheet music, if you're so inclined.
Check it out!