PhoneZoo is a ringtone service that lets you make your own ringtones, download them to your phone and share them with your friends. The site smartly gets around copyright issues by having users create ringtones using songs that they upload.
PhoneZoo has two kinds of ringtones available for sharing. They have public domain audio files that can be downloaded by anyone and they have user-created ringtones. If you want to download a user-created ringtone you have to first upload the very same song, which again helps to get around copyright issues.