Oh man, several have freaked me out.
1) When I was really little I dreamed that I got out of bed and sat on the can to take a dump, and this little gremlin creature came out of the toilet and clawed chunks of flesh out of my butt. That image is still scary.
2) In high school I was having some troubles in a relationship (if you can call them relationships at that point) and had a few strange dreams, but one really stands out. I was walking for what seemed like days on a dry beach. It never became light out, and I never heard or felt the water; it was a completely still ocean. Finally I saw this run down house on a dune and went inside. I stumbled around in the darkness until I saw a door with light in the cracks at the top of the stairs. I opened the door and saw my then-girlfriend sitting perfectly straight up in a wooden chair in a spotlight with her back to me; everything became too dark to see except for this chair in the spotlight. I called out to her--nothing. So I walked over to her and looked at her--she had no face, just flat skin where her features should have been. I recoiled in terror, and her head quickly jerked up to "look" at me. I woke up screaming. We broke up the next day.
3) This week I had an exam at 9:30am one day. My girlfriend woke me up at 8am before she left; I remember this and she said I had been sitting up. I must have fallen back asleep. I dreamed vividly that I got up, got ready, rode my bike to university, and walked up to the room where the exam was. I stopped just before entering, though, took a deep breath, and looked straight up. At this point I woke up in my girlfriend's apartment, looking at the bright red alarm clock numbers reading 9:46am. Fuck.