My son signed me up to ringtone maker, Mobile 17. it's perfect! I can use songs from my itunes or other collections and adjust start and stop points and use up to 40 seconds. I have had this phone over a year and a couple weeks ago my son heard me grumbling about the latest batch of useless ringtones and made it his mission to try to hook me up.
In addition to the tool that bingcrosby mentioned, I found a great website to use. It's free to register and use. You can create your own ringtones, and the site sends it via text message to your phone. I've been using it for about 3 years now and I love it!!!
Check it out!!