The history of iPhone ringtones has been tumultuous. First there were no custom ringtones whatsoever. Then a number of third-parties introduced methods for moving ringtones to the device with or without hacking. Apple then introduced a scheme for buying ringtones through iTunes, simultaneously thwarting some third-party efforts. Finally, built a routine into its GarageBand application for Mac OS X allowing custom ringtones to be sent directly to iTunes. See our guide "The many methods for using custom iPhone ringtones" for detailed instructions on all of these methods.
Now a company called Nakko, which offers a variety of free ringtones for mobile devices, has introduced an automated Web-based delivery scheme for iPhone ringtones that -- in initial testing -- works well and doesn't require any iPhone hacking. The service works by maintaining an online repository of ringtones defined by the user, then pushing a podcast to iTunes that automatically adds selected tones to the application's "Ringtones" section. You can test an example ringtone here.
Once you've created an account and a "locker" with Nakko, you can add ringtones from the site's collection. When viewing your locker, you can then click the "Synchronize with iTunes" button to automatically trigger delivery.