Custom Cell Phone Ringtones, Free!
Many people still don't know that you can use mp3's from your computer as free ringtones. I'm going to take it a step further and show you how to crop a certain section of a song to use for a ringtone.
To begin, download the audio editing software called Audacity. It's 100% free and well worth every penny! Thanks to the developers for making this amazing software free.
You will also need a DLL file called Lame Encoder (lame_enc.dll). This DLL is required to encode your new mp3 file.
Once you have installed Audacity and downloaded lame_enc.dll, put the DLL somewhere where you won't delete it (C: or your Program Files folder are good options). Now on to the editing!
Open your file and press spacebar to start playing the song or look for the spikes if you know where you want to start from. In my case, I know roughly where I want to trim and I can also see it here:

Once you have determined where you want to start it, click and highlight everything before the spike and hit delete.

Do the same for the ending. I try to keep my files to about 30 seconds. This not only keeps the file size down, but it will also load faster on your phone when somebody calls. And really...who takes longer than 30 seconds to answer a phone?

Now for the final touches...
If your song keeps on going for more than 30 seconds, you don't want it to just end abruptly. Highlight the last 3-4 seconds of your clip and hit Effect > Fadeout. Repeat this step as many times as necessary, don't be afraid to fade out a part that's already faded out. Experiment!

Now we need to export the file and save as an mp3. In the top menu, hit "File" > "Export as MP3...".
You should be asked to locate the lame_enc.dll file you previously downloaded. Follow the program's requests and you should be good to go!
If the file quality doesn't sound very good, you can adjust the bit rate in the preferences by choosing "Edit" > "Preferences" (or CTRL + P). Then select the "File Formats" tab. Here, you can also select the lame_enc.dll location before-hand if you want to skip a step.

Good luck!