I use KavaMovies on my Hackintosh.. Its the best I've seen so far..
What you need to do is just browse and show your movie folder to KavaMovies and it'll do the rest.
It'll not only download DVD Covers for all your movies but also info like Year, Genres, Keywords, Languages, Release Date, Rating, Runtime, Plot, Cast, etc..
It also lets you add your additional personal info in it like Format of movie, the date you've seen the movie, where have you seen the movie, YOUR Rating, your comments,etc..
Moreover, it also lets you organize your movies in categories like "Want to see, Don't want to see, Seen it, Haven't seen it, Got it, Haven't Got it".
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And all this info is stored in a file..