Hm. Interesting question...
I've had many nightmares; one of the most frighting was one with me, in my room (different house at the time), and it filling up with water... EVERYTHING filling up with water. The space in my room, and outside... (similar to the game Rascal) Anyway, then the walls start closing in on me, and I run to the door, but it's painted on (no, that isn't funny =P).
A more complicating one that has stuck to me till this day:
I was in 3rd grade, and I was just sitting in class, doing nothing, just sitting. After a while, I get suspicious, as no one is moving! I look outside, and it's a nice day... but everything is going at incredible speeds, and changing. But, I noticed something... everything was going in reverse! I looked at the clock and it was going counter-clockwise, and after a while, it just ended up looking glitched, which scared me because I was (and still am) an avid video game player. All of a sudden, I'm still sitting at my desk, but everything around me is reversing... ceasing to exist. Then after a while, a map comes out of no where, and is chasing the people I knew and loved; circling around them, and finding that they had disappeared. I knew I was next, but I ran like hell anyway... it caught me, and in a matrix-like fashion, I said, "Nooo!" (slow-motion, lol). I was in another dimension; trapped as a being that was only a pair of floating glasses, that played a glitchy tune whenever it moved. Not only that, they don't breathe, so I stopped breathing in my sleep, and woke up on the floor.
Although, nightmares aside, I love the ability to dream; don't take it for granted! I've had some amazing dreams that influenced who I am to this very day, such as: Dreams where I'm just flying in the clouds over places I've been, or haven't been (usually during the nighttime), dreams that become exactly what happens the next day, or some time in the near future, and these really odd dreams I've been having since I can remember, which are all brownish gray, and I'm this lizard-like thing...
At any rate, dreams are cool. I wouldn't be able to live without them.