Starting Points

Internet Movie Database : The ultimate source of all movie information.
Allmovie: For when you're not in the mood for IMDB.
Movie Review Query Engine: Find almost every major review available for any reasonably current movie.
Rotten Tomatoes: Think the Movie Review Query Engine but with reviews categorized by rating and in tomato-form.

Critics

Roger Ebert: The Chicago Sun-Times' great grand-daddy of movie critics.
David Edelstein: Formerly of The Slate, now of New York magazine.
Anthony Lane and David Denby: The New Yorker's Film File.
A.O. Scott: The New York Times.
James Berardinelli: Reelviews.
The Onion A.V. Club