Magic Eraser uses... did you know you can cut a piece off and float it in your toilet overnight and voila! no more toilet ring! The website has a list of about 100 uses!
Magic Erasers.. they clean like magic. I honestly love these things and use them in place of typical sponges on most everything. (Though I personally prefer the generics.)
But will they be so magical as to do the work for you?
No. No they won't. You can't just cut up a sponge, toss it in the toilet, and expect to wake up to having the bowl cleaned for you magically over night. The way these sponges work are that they are basically like super fine sandpaper (which is why they tell you never to use on skin. Though I'm surprised I haven't seen it posted a great DIY micro dermabrasion facial on pinterest yet.) So you actually need to scrub with these. Plus you still need to fish that out of your toilet come morning anyways so wouldn't it be easier just to clean it the old fashion way?
Honestly if you keep your toilet fairly clean then in a pinch you can add anything from your usual bowl cleaner, bleach, or even baking soda and let it sit over night and then just flush in the morning to get rid of the ring. I've also read where people have had good results with the old 3rd grade science fun of mixing equal parts baking soda and vinegar to clean with out scrubbing. However if you have hard water or mineral build up that isn't responding to the usual cleaner and brush it's best to slap on the industrial rubber gloves, grab a pumice stone and get scrubbing.