This is one of the reasons I'm a massive fan of JustWatch[0]. The other reason is that it searches across everything that's available on other services in Australia; most of these tools ignore us Down Under!
You can download the IMDb rating database, although I think it has a very restrictive licence. Alternatively there is this: http://www.omdbapi.com/ - but I wouldn't count on it being around forever. It seems like they would be on shaky legal grounds (but at least it would be them getting sued rather than you).
That website is how I discovered netflix has thousands of movies in the first place.