Via KK.org’s cool tools section.

A fellow by the name of Jeff Lindberg has thought of an ingenious way to get rid of those tough stains in your toilet bowl. His suggestion is to grab a cup or two of white vinegar and leave it sitting overnight. Come back the next day obviously and a minute or two of scrubbing later, the stains are completely gone. I personally haven’t tried this but I remember growing up, my mom would use vinegar in her cleaning arsenal and it seemed to be relatively effective. So maybe it’s worth a try.