Paranormal Activity 2 - It's a sequal that's a prequal!
While I generally don't like spoilers, I will say that PA 2 is both a prequal (which is the main chunk of the movie) and a sequal. I for one very much enjoyed this second installment in the Paranormal Activity franchise. They stayed true to the original, while still branching out and trying different things. A major difference is that in this number 2 there are multiple different cameras throughout the house, plus a handheld, giving you the ability to see different rooms in the house, while also eliminating a good deal of shaky cam, if you don't particularily like that shooting style then you'll find PA 2 much more palatable then the first.
I say you should see this movie especially if you saw the first, because as I said before, this movie provides both backstory, and a continuation to the original.