So, at the mention, I remembered I wanted to look at that. Wikipedia has a really interesting article on the history, workings and criticisms of the MPAA ratings system. I'd mentioned to SPC that with all the criticism, surely someone's started alternative ratings systems by now. He was highly skeptical. Well, I win.
(By the way, some of the links below offend my religiious sensibilities, but I include them anyway, as some parents may find them useful.)
Film Advisory Board
United States Conference of Catholic Bishops' Office for Film and Broadcasting
That's nice to know, but I still don't find these ratings schemes entirely satisfactory, as they don't take into account what kinds of content are playing into the decision for a particular rating. For instance, I don't find huge amounts of violence with the blood and gore omitted to be "okay". I don't like that kind of lack of consequence in live action film. So a movie that gets a mild rating based on that is going to tick me off.
Here are some systems and sites that dig a little deeper:
-- How motion pictures are rated 'round the world
-- Television rating systems
-- Video game rating systems
-- Marvel's comic book rating system