MakeiPhoneRingtone turns AAC files into ringtones for your iPhone. Mac OS X 10.4 or Later
New in 1.3 - Now works with iTunes 7.5 and iPhone OS 1.1.2 (both required)
MakeiPhoneRingtone is a quick drag and drop utility to turn AAC files into ringtones for your iPhone. Make ringtones from audio you already own, instead of purchasing it again and again.
OSes: OS X 10.4 - 10.5.2
Architecture: PowerPC, Intel
http://rogueamoeba.com/freebies/