Instead of writing up my own explanation of what Metacritic.com has to offer and why I like it I will let it speak for itself.  Here is the description straight from the site.

“Metacritic compiles reviews from respected critics and publications for film, video/dvd, books, music, television and games.

Our unique Metascores show the critical consensus at a glance by taking a weighted average of critic grades.”

Much like Rottentomatoes.com metacritic has the critics score as well as an average rating from visitors.  Unfortunately the sites back catalog of reviews is a little light, as they only started compiling info on new releases as of 2000.