There are two free film-review databases you'll want to know about, Movie Review Query Engine and Metacritic.
MRQE is the more undisciplined of the two, including a wide variety of sources from the credible to the somewhat bizarre. It also includes several precomputed lists of films, the most interesting of which are the listings of titles from major film festivals from the last few years.
The Metacritic database is more refined, focussing on reviews from better-known publications and critics. Reviews for a given title are ranked in order from positive to negative. Each title is given a Metascore based on a weighted average and the score is correlated to a green-yellow-red system, so one can see at a glance a consensus opinion on a given film. Of movies now in theaters, Brokeback Mountain scored the highest at 87 while Date Movie managed to eke out a mere 11. Metacritic also has sections for reviews of DVDs, books, music, and computer games.
The strength of both sites is on films released in theaters or on DVD since the mid-1990s.