Any number of Apps that will allow that. Search the App store. The trick is you have to copy the ringtone from one part of the phone to another part thru iTunes and you have to remember to rename it from {filename}.M4A to {filename}.M4R so that it works.
Long as you follow the directions of the App you use, then you should have no issues. BTW, if you avoid iTunes then you have to jailbreak your phone to do it.