Although I've showed you several ways to make your own ringtones previously using some pretty cool applications, and you can make almost any song a ringtone in iTunes for a fee, why not use some of the music you already have and create custom ringtones on the fly? This is especially for you Mac users, since it uses the apps you already have stocked in your machine.
All you need is the latest version of Garageband (4.1.1), iTunes (7.5 or later), and some music, and you're good to go!
Check out my slideshow for a step by step guide to making your own ringtones. . . for free!