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Mobicious, the online resource for all things mobile, now has a free service for creating ringtones.
Using an mp3 from your desktop, you can choose a 20 second clip for Mobicious to convert into a ringtone. Send it to your phone, and you've got a free and easy way to personalize your mobile. The good thing about Mobicious' service is that you get to pick which part of the song you'd like to make your ringtone from.