Audacity is an audio editor that is free and easy to use for both Mac OS X, GNU/Linux and Windows. It is one opf very few applications that can be put onto a USB. You can trim, change the speed or pitch of the sound or even record yourself. You can also make a ringtone, which I have done today.

Steps:

1. Open Audacity then go Project > Import Audio then select whatever song you want.
2. Trim the part of the song you want to use for your ringtone by selecting the part you want then going to Edit > Trim.
3. Go to Effects > Equalization
4. Follow the steps on http://manual.audacityteam.org/man/Tutorial_-_Making_Ringtones for Equalization.
5. Go to File > Export WAV and save it.