For obvious reasons, I don't cover ringtone generators, but I have to make an exception here! This particular ringtone generator is from, Oliver Wittchow, the original developer of nanoloop for the Game Boy!
nanoloop R is a stripped down version designed for making ringtones.
How stripped down? Completely! There is nothing you can do with this, other than hit the New button and hope for something useful. It'll generate two tones and run them through a fast tremelo. If you do get something you like, you can save it. Since Apple won't let you install ringtones from an app you'll then reimport them to your iPhone through iTunes.
All of this seems like a terrible idea, but once again, original developer of the Game Boy nanoloop! This guy is so smart he even knew this was a bad idea. Instead of just releasing this app, he has also released a free version you can play with on the web: http://www.nanoloop.com/r/
You can't buy nanoloop R on iTunes: Because they yanked it just as I was about to post this!