Information on Objectionable Content in Movies
Kids-in-mind.com has the easiest rating to understand. It uses a three number system to rate the amount of sex, violence, and language (in that order) that a movie contains on a scale from 1 to 10 (10 being the most offensive). For instance, Harry Potter 6 received a 4.6.3 rating.
Screenit.com is similar to the previous one, but has way too many ads. Like Kids-in-mind.com, it can sometimes be too detailed and you can get a false perspective of the movie by focusing on all the negative elements lumped together. They're (kids and screenit) usually right on when they rate movies as very offensive; it's the middle ground one's that they can sometimes be too picky on and take things out of context.
CommonSenseMedia.org gives a much higher level view of the content to be careful of and tries to match the age recommendation a little more precisely. In addition to violence, sex, and language it also rates the amount of drug use. This one along with kids tries to tell parents how they can talk to their children about the content they view in the movies.
USCCB (United States Conference of Catholic Bishops) is a moral rating website (most similar to this one) that rates movies based on their moral value. It categorizes the films into those appropriate for all ages, those for adults and adolescents, and those for adults. After that you get movie types with content that could be be "troubling" to even adults and then those that are just plain morally offensive. Reading through their reviews, they don't tend to be as strict as we are and their reviews cover a much more comprehensive list of movies.
I don't believe it is necessary to see every movie and help others understand it's moral content. Most movies you can tell whether or not the content will be morally offensive just from the trailers or the actors (e.g., Adam Sandler or Jack Black) in the movies.
IMDB.com is an alright place to get info on movies. Their warnings on the content are user contributed and are sometimes just a cut-and-paste from one of the above sites. They provide a way to keep track of movies you've seen and allow you to give and track your own ratings. Its main purpose is as a database to find out actors and the movies they've been in, directors, producers, etc.
RottenTomatoes is the best I've found at aggregating film critics reviews and then using those reviews to categorize the movie as good or bad (giving it a fresh or rotten tomato). If 60% or more of the critics give it a fresh rating, it remains fresh; anything less than that is rotten. They don't single out the objectionable content. At most they throw in a blurb on why it was given the rating it received from the MPAA and then sometimes the critics will point out any unecessary negative content.
Some other sites that are worth a look into are: