Not a movie, but a series on TV:
The Sharpe series.
I thought it was really good - Sean Bean was excellent as Sharpe. I particularly liked Posthwaith (sp?) as the evil and crazy villian, talking to his mother in his hat. 
Duece Bigalow Male Gigalow
WAY better than expected.
Catch me if you can. I went into the theater planning to hate this movie, I was amazed at how good it was. Spielberg is not a good story teller [IMHO], but his visual skills and a brilliant script overcame his tendency to to be sappy. DiCaprio gave a great performance, as did Tom Hanks. Just seeing Christopher Walken as DiCaprio’s father is worth the price of admission.
Drowning Mona. I watched it on cable one day, and was pleasantly surprised to find it’s a quite well done black comedy.
Another vote for Super Troopers here. The ads made it look like a horribly stupid movie, and it is, but it’s damn funny. When we went to see it, we never expected to be laughing with it. Especially the “Afghanistanimation.”
And Lilo & Stitch, too. I’m a big Disney fan, although I’m beyond tired of the offensive Disney formula and marketing tie-ins and such. So I go see all the movies but dread having to wade through all the crap to get to the good stuff. I put off seeing Lilo & Stitch because of the marketing campaign; the cute marketable character with the annoying Howie Mandel voice running around showing how zany and irreverent he was. And they even played BAD TO THE BONE in the ads, which is even worse than starting out with “In a world…”
When I finally saw it, I was surprised to see that they’d jettisoned the Disney formula except for where it made sense. The characters are genuinely endearing, interesting, and dysfunctional instead of being cloying. The crises are real and believable. The music was well-used. The character design & backgrounds were unique while still being appealing. The humor was genuinely funny and even more surprising, subtle! And most surprising of all, Stitch wasn’t a cute, irreverant troublemaker with a heart of gold – he’s really a total bastard for most of the movie! Amazing that a Disney movie was able to get greenlighted even without the musical numbers, the wacky sidekicks, and the same old tired formula that’s been beaten to death since The Little Mermaid.
Meow.
Fargo. I was expecting it to be depressing, but it turned out to be hilarious. I will never look at a woodchipper the same way again.
That’s amusing, because the exact backwards thing happened to me: I went in expecting it to be funny, and it depressed the hell out of me.
For me it was The Shipping News. Based on the promos, I guessed it was a movie that Grandmommy might enjoy – Judy Dench, Kevin Spacey, New Foundland, boats etc.
Crazy events unfolded (like a Lucy episode) and I was “forced” to watch it. Well I liked it. Alot. Top 25 all time Movie? no way. But it had alot more “story” and grittiness to reccomend it. I am glad I saw it.
I’ll second the votes for Matrix because I usually don’t expect much with Keanu Reaves. I’ll also second the vote for Galaxy Quest because I expect even less of Tim Allen.
Two others Pleasantville turned out to be twice as smart as I thought it would be; and Babe . I was going to watch about 5 min of that pig movie, while the kids got settled and then split, but I watched the whole darn thing. Well done.
Galaxy Quest
Last of the Mohicans – when the primary scene that stuck out from the trailers was the cheesy under-the-waterfall “I will find you” bit, I didn’t expect it to be as good as it was. I thought it was wonderfully done, and the musical score is incredible. One of my favorite movies.
American Pie I thought this was going to be a sub-moronic teen comedy. Jesus, they not only have the pie sex scene, they named the whole movie after it! But it actually had interesting characters and a surprisingly adult attitude towards sex, once all was said and done.
Pretty much ditto for Friday, which I expected to be more like, well, its own sequel.
Additional votes for Oscar and Galaxy Quest.
A Beautiful Mind
and I agree w/Fight Club
I know its been mentioned, but i think Two Towers deserves my vote. You see, i half expected to go into that movie feeling underwhelmed. When i read the lord of the rings a year or so ago, i remember vividly imagining every battle and every scene specifically from Two Towers to Return of the King. When i first saw the scene with Gandalf falling/fighting, my jaw hit the floor in a way that a child’s would when opening presents on christmas day. It turns out, i liked this movie more than anything i have seen yet.
Also, About Schmidt is a really good choice as well. I went in there and was the only one in my generation. The rest were all beyond their forties a good few years. I thought, “Oh my god, this is going to be way over my head.” Turns out, that movie was on par with A beautiful mind and all those other really good thinker/emotional flicks…
Myself and others have been pleasantly surprised by Dude, Where’s my Car?. I wasn’t even high at the time.
I also give a thumbs-up to Goldmember which I was expecting to suck, it being the third movie and all…but it was really funny!
I enjoyed X-Men more than I expected to. I knew nothing about the story going in, and I was afraid it was going to be filled with all sorts of “inside” references, but was pleasantly surprised to find it was a perfectly enjoyable little sci-fi movie, on its own terms.
Another me too on “Galaxy Quest,” which looked stupid but was actually quite clever and one of the funniest movies of the last ten years.
This might surprise people, but the original Twelve Angry Men had a much greater emotional impact upon me than I’d expected it would. Usually when I watch “classic” Hollywood films, like Casablanca or The Third Man, I feel like I’m viewing them at an incredible distance - I can appreciate them as works of genius, as historical documents, but I’m rarely completely drawn into the movie’s emotion.
But I was totally mesmerized by Twelve Angry Men. In a sense, it was the first pre-1970 film I’d seen that didn’t feel ever-so-slightly like homework for a film class.
Oh, and I’ll also cast another vote for Dude, Where’s My Car. Ostriches make anything funny.
Tremors
Matrix
And believe it or not, Star Wars Ep. IV: A New Hope.
I’ll second this entire post. 300 Miles to Graceland was truly, truly horrible. It must hold the records for the most endings in one movie. The first one is about 1/2 way through the movie!
Fight Club and Galaxy Quest are 2 of my favorite movies.
I was surprised at 8 Mile - Eminem can actually act (as can Kim Basinger)
I’ll add the remake of The Thomas Crown Affair too. Didn’t expect a great deal but really enjoyed it
Independence Day. It just gave me a warm feeling about modern life, in particular technology (jet fighters, laptop computers etc) and the adaptability of humans to adverse circumstances; a feeling that persisted for several months after seeing it.
To everyone else I know, it’s just a B-movie. Sigh.