One of the best is Classic Cat. It is a frequently updated database with links to other websites where the MP3's are stored. Look up a composers name, and you will find all the works that are available for download. Then there is free music from the US Air Force Bands here. They have a lot of great marches and patriotic songs, classical, jazz, and ceremonial music.
If you need some help teaching your classical music units, turn to Classics for Kids. You can read and listen to stories about the composers, and hear streaming audio of some of their great works. Dallas Symphony Orchestra and San Francisco Symphony have good websites with games and activities for kids.