I have a Mac and have been using:
Rogue Amoeba - Under The Microscope
What I do is fire up Garageband, cut out the clip of a song I want, do any fading in and out then export
song (NOT RINGTONE, they sound terrible) to iTunes. Then right click that song and click Convert Selection. After, I locate the converted file drag it in the app and instantly converted and it shows up in the Ringtones section in iTunes.