I had this experience in July 2008. I still don't really know what to make of it. I have my own ideas, but to avoid influencing anybody's comments, I won't state them here. A little background about myself first, I have never experienced a ghost, alien, angel, or anything like this prior to this event, although I do believe that they exist.
Well, I woke up between 3-4 in the morning, and a few seconds after awaking I felt the sensation of something gripping my heart and squeezing it to the point where it was painful. For me to admit pain, actual pain, with what I've gone through in terms of getting hurt playing sports, means that it was really something bad. Also, I don't wake up at those hours of the morning anymore without a reason I'm aware of, a dream, having to go to the bathroom, or something along those lines. When I woke up for this, there was no immediate cause that I can remember that forced me to wake up... All I remember is waking up.
In the corner of my bed room, there was a black shadow, darker than my room that appeared to be humanoid, but somehow didn't really have a shape. I never looked directly at it, but only looked at it out of the corner of my eye, which is unusual because I ALWAYS look at unusual objects/things. I do remember it seeming to reach out toward me at one point. All I did was curl up into a ball and started praying. A minute or 2 after I started praying, I felt the pressure in my chest release, and with it came a profound calmness I've never had before, and I've managed to maintain a piece of that calmness ever since.
My house is not haunted, nobody has ever died in it, and there's no ancient Indian Graveyard underneath it. I have no explanation for what happened.