Needless to say, Paranormal Activity is doing very well, having grossed $97 mil in less than two months after creating the film with merely $11,000. Even the actors are profiting, getting an upgrade from their original $500 salaries.
I haven't jumped on the bandwagon as of yet, because I can't watch horror movies, but after reading the EW article on how the film got made and distributed, I stumbled upon not only the ending, but the alternate endings.
In the original ending, the police try to arrest Katie after they find Micah dead. But when a door slams, they get startled and shoot her. Two other endings involved Katie killing Micah, then returning to the bedroom to slit her own throat with an evil grin on her face in front of the camera, and Katie bludgeoning Micah to death with his camera.
The ending that's in theaters was actually suggested by Steven Spielberg because "he didn't think [it]delivered a big enough payoff," despite the fact that he could only watch it in broad daylight. lol Wow! If you can scare the guy behind Jaws, then you might actually have a future in this business.
If you'd like to know the rest of the film's backstory, check out the EW article.