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www.themoviedb.org is pretty damn good and powers a lot of the homebrew media center solutions.

Edit: If it is critical to your app, you might want to consider licencing IMDB http://www.imdb.com/licensing/

http://www.omdbapi.com/

http://developer.rottentomatoes.com/

+1 on rottentomatoes. easy to get an API key with reasonable rate limits for free, and the API has a decent fuzzy search. (you might have to do some fiddling with the results, but what you're looking for is usually there)

I used IMDB several years ago, but now see that their usage policy is restricted to "personal" use. Who is going to download 1.11 GB of compressed text for personal use?

http://www.imdb.com/interfaces

Related, if someone knows of an API to get commercial links (buy/rent) from tmdb or imdb ids, it would be useful for me.