An interesting conversation in my Reference and Information Systems class arose in response to one of our homework exercises in which we were asked to find a review to the VHS release of the 1927 movie "The Prairie King". Most of the students had been stymied by this one, save for the film geeks, who recommended a slew of online indices for films and film reviews that haven't quite made their way into the Reference librarian textbooks yet:
Internet Movie Database - the first of its kind, and definitely the most comprehensive. IMDB was recently acquired by Amazon, which should only further raise its profile, but the question remained for us: how authoritative is it? The consensus seemed to be that it is the most authoritative of the online film indices, but that isn't necessarily saying much.
Rotten Tomatoes - offering a clearinghouse of film reviews, from the national news networks to the local papers, Rotten Tomatoes is not only a movie database but a fairly comprehensive review index as well. The signature gimmick of the site is its overall rating metric, which samples a hundred different reviewers (the same for every movie) to determine whether or not the movie was a rotten tomato.
Metacritic - I wasn't aware of this site until one of my classmates swore by it, but it's supposed to take the Rotten Tomatoes concept and extend it not only to film, but video and DVD releases, music, and even video games. I'm not sure how comprehensive the site is compared to RT or the IMDB, but the concept is sound and the interface is slick, so I'll be sure to give it a try.