(Thread prompted by the one about seeing someone with lizard eyes, who is possessed by a demon)
I have once. Waken from a dead cold sleep, my girlfriend and future wife said to me "we're not alone". I looked at the foot of the bed and their was a very angry young boy. I was shocked and became afraid myself as I sensed a power there, and then, like a balloon popping, he vanished.
She asked me what I saw, I said a very angry young boy, who looked to be age 9-10. She confirmed that was what she saw too.
I suppose it could have been an angry guardian angel, or even a passing demon, but let's go with ghost.
I have never doubted things spiritual since that day. I was about age 19.
When one of my grams was dying in the hospital I was home trying to sleep (sometime after 11pm) and I sat bolt upright and couldn't catch my breath, got a phone call within minutes she had passed. My mom was in the room with her. I still think it's weird.