Ambrosia Software has released iToner, a Mac OS X tool that allows you to transfer custom ringtones for your iPhone - without modifying, hacking or having to reset your iPhone. You drag and drop MP3 or AAC audio files on iToner's window, click the Sync button, and the items are automatically moved into the iPhone's ringtones section. Ambrosia says the ringtones will continue to work with future iPhone OS updates.
We've been playing with beta editions of the software for the past several days, and are generally impressed with its functionality -- basically it's a simple tool that works as advertised. The only real limitation is an inability to modify MP3/AAC files on the fly, selecting the portion of a track you actually want to play as your ringtone. For that you'll need a tool like Rogue Amoeba's Fission, Amadeus or another sound editor. Also note that if you are willing to hack your iPhone, there are free, but slightly more complex solutions (see "SendSong" in our native applications guide) for accomplishing the same thing. From the standpoint of simplicity, however, iToner is it.
The software is available as a 2.7MB download and is priced at $15. There's a 30-day trial mode.