www.mutopia.com is excellent.
www.sheetmusicarchive.net. (This one only allows you to download 2 times a day, but sometimes it messes up and doesn't let you download at all. If this happens, just clear your history or cookies and it should work.)
http://www.free-scores.com/index_uk.php3 (I only recently found this one recently and haven't used it much, but I was pleased with the results.)