Create ringtones for iPhone on Mac OSX (for free)
A simple app ought to do it.
Download iRinger. It used to be a Windows app, but is now available for Mac OSX!
A very intuitive application. Select a song (mp3 for example), it clips it and shows you the ringtone-worthy segment of it, you basically export it as a ".m4r" file.
When done, just open the file and iTunes will automatically open it. Done.
If you want to do this without an app at all, just online, then here's a great website.
http://audiko.net/