I like The Internet Movie Database (IMDb), since it is helpful in finding out who starred in which movie again. It can also be helpful to determine whether a downloaded movie is the correct length.

Most countries have a rating system for movies, to indicate whether or not a movie is suiteable for certain ages. The US has the movie rating system of the MPAA, while the Netherlands has the Kijkwijzer. In addition to merely putting a suitable age (group) on a movie, both also offer explanations of why a movie is unsuitable for other groups.

Since the IMDb contains the rating information, it would be helpful if it provided a search function that allows one to search the ratings. In particular, a function that allows one to search for movies not containing a certain reasoning. It could be enhanced by extra information that might be a reason for people not to watch a movie. I'm thinking of things like “(an abundance of) spiders”, “rats”, “naked George Clooney”. This way, people can search for movies they may be interested in, while filtering unwanted elements.