One way to do it is to use the microphone in your phone to literally record the song and then send it to yourself and download it as a ringtone. It's kind of a pain and the quality isn't great but it is FREE.
Click on this link. You have to have whatever song you want on your computer though and you can decide how long you want the song and where you want it to stop. It gets sent to you as a text message after you've made it and in the text it'll have a link that will take you to the internet to download it from your cell phone. But this works great!
