Okay, the science didn't quite add up, but Signs is still a brilliant movie and very terrifying. Graham Hess was a priest until he lost his faith when his wife died in an accident. He lives with his son, daughter and brother on a corn farm. One day he finds mysterious crop circles in his cornfields. Then alien craft appear over the world's cities.
The minimalism of this movie is what makes it so terrifying. Graham and his family are trapped in the house, their only source of information a book and the media. he threat is only hinted at, rarely explicit. It is the first movie in a very, very long time where I have felt like hiding my eyes through fear (as opposed to sympathetic embarrassment - see Hugh Grant movies). It's like a nightmare, the feeling of no control over events.
The acting, directing and photography are all fantastic, and James Newton Howard's score is effective in it's absence through much of the movie - the silence is more powerful. There's humour - not the crass humour of most horror movies that comes through laughter at stupid props - but gentles humour that adds a human dimension to the characters without destroying the tension.
Don't watch this movie with a rational mind. Just surrender to your fears.