The Guardian - Film Reviews
The New York Times - Film Reviews and Archives
Rotten Tomatoes - Film Reviews
(Pay attention to who the reviewer is - it makes a difference!)
Metacritic - Aggregate Film Review
(Search for a film; use the "Critic Review" tab for the most reliable sources)
Variety Magazine - Film Reviews
Rolling Stone - Movie Reviews
AMC Filmsite - All things movies.
Roger Ebert Reviews - Continuing review site in honor of famed critic Roger Ebert
Movie Review Query Engine (MRQE) - Conglomerate/one-stop-shop for links to all levels of reviews.