RingtoneFeeder supplies your iPhone with ringtones
From there, you can easily make a Smart Playlist (kind = ringtone) and you're all set. All of the ringtones you'll receive from RingtoneFeeder are created by Geoff Smith, whom you may remember from Adam Curry's Daily Source Code Podcast.
Of course, you can create your own ringtones for free (what's the difference between a "ringtone" and an audio file, anyway?). For example, create a file in GarageBand (40 seconds max) and select "Send ringtone to iTunes" from the "Share" menu. Note that you can only use unprotected audio files for this process (Unless you strip the DRM. We would never suggest doing that!).
For those of you who couldn't be bothered, check out RingtoneFeeder.