With the return of methods to create free ringtones on the iPhone, RougeAmoeba has reintroduced their MakeiPhoneRingtone application which automates the manual process of renaming your AAC files and reimporting them into iTunes.
Drop any AAC file on MakeiPhoneRingtone and it will be processed and passed off to iTunes, where it will be handled as a Ringtone. The file will show up in iTunes' Ringtone sub-library as well as the Ringtones tab in the iPhone section, where you can sync it to your iPhone. Once it's on your iPhone, you can use the file as a global ringtone, per-contact ringtone, or even an alarm clock sound.
The application is only available for Mac.