I usually visit the site rottentomatoes.com. Its excellent for Hollywood movies, and international movies that do well/badly in the US. I always read reviews on this website before catching a movie. And the best part is that because movies have a premiere before they are actually released, critics write their reviews here before the release, and so one can decide to watch it or not.
But of late, there has been a downside. I do not agree with the critics. They tend to overrate movies that may just be sob stories, and underrate commercial movies. Moreover, sometimes just to play up a movie critics may give it a higher rating before the release, and after the movie release the ratings may gradually keep going down as more and more critics add their (negative) reviews. I have noticed this aspect only recently. I guess its because movies were not doing so well in the US since people followed the reviews!
Anyways, I like this site and tend to check out user ratings too, which I have found very helpful. In fact, when I check out a users ratings for a particular movie, I can also see other movies he/she has rated. If he/she has likes/dislikes similar to mine, I follow his/her ratings, and that has never let me down.
One word here, though. Since Americans have a slightly different sense of humor than we Indians, one may want to not follow reviews of comedies as religiously. School of Rock was a big hit with both critics and users, but I could not really follow the kind of humor in the movie. Maybe Americans can relate more to that kind of stuff.
Anyways, I do like the website, and just wanted to share it. Another website is imdb.com, but I find it very confusing because of its huge database of movies.