Pocketmac RingtoneStudio for iPhone is the latest tool released by Information Appliance Associates which allows you to convert almost any media file into a ringtone for your iPhone (PRmac).
Pocketmac RingtoneStudio works with media files such as sound files, iTunes songs, and Mac friendly video files to produce ringtones that can automatically be synced to your iPhone. One of the many powerful features of the application is that it allows you to choose any 30 second snippet from the media file to use as ringtone by using its EditingBay feature.
The Pocketmac RingtoneStudio works with OS 10.4x and 10.5x (Tiger and Leopard) and should retail for about £8 ($15). You can read more about the Pocketmac RingtoneStudio here.
Read more about price and product features for new 3G iPhone here.