This happened at my parents' house when I was a kid in high school. I have to give a bit of background and house layout information first, though; it may seem unrelated, but it IS relevant to the outcome of the tale.
My best friend in high school was over for a visit one day. We were just hanging out watching television in the living room. I was seated in my Dad's chair in the corner; my friend was seated on the sofa directly in front of the television set. At the far right end of the living room was a doorway that led to the front parlor. If you exited the living room from this doorway and turned left, you would see a second doorway to a hall that led to our kitchen.
Anyway, we were watching TV and screwing around when I left to go up to the bathroom. I returned shortly to find my friend at the bottom of the stairs, coat on and ready to leave! I asked her if she was indeed leaving, and she said yes, she had to split as she suddenly wasn't feeling well. She certainly seemed agitated, so I just said OK and told her I'd see her later.
Shortly after this visit my friend and I got into a fight over something and weren't speaking to each other for about 1-2 weeks. As a consequence of this, one Saturday afternoon I was sitting by myself, bored and miserable, watching TV when I would normally be hanging with my friend. I was sitting in the same position on the sofa as my friend was when she had last been over, checking out the World Wrestling Federation or some such, when out of the corner of my eye, I saw a man walk from the front parlor through the doorway that led to the hall that led to our kitchen. Once he caught my eye, I had just enough time to turn my head and get a good look as he moved beyond my sight. He was looking in my direction as he passed, appeared to be around 30-35 years old, was tall and lanky and a bit stooped, had ginger-kid coloring, wore a chambray work shirt with the sleeves rolled up and Dickies, and had what looked like a tool belt hanging around his waist.
When I saw this guy, I felt no alarm. It seemed like seeing this guy was the most natural thing in the world. He totally looked natural, no transparency or misty aura or anything about him. After a beat, I realized that this guy is a stranger, what's he doing in my parents' house? I then jumped up and went around the corner to see where he went. From my seeing him to my going into the hall took all of 3 seconds, but by the time I got there, the guy was gone!
Then I got spooked! In fact, I was so spooked; I broke down and called my friend, the one with whom I was not speaking. With trembling voice I told her that I saw a guy in my house just now, and asked if she and I could please set aside our differences because I was freaked out and needed a friend. She said then that she had also seen a guy in my house, that day she was over watching TV, and that was why she had left in such a hurry! She didn't tell me what she saw because she didn't want to alarm me, but she felt that she couldn't stay, as she was rather spooked by the incident. Without telling her the specifics of what I saw, I asked her to describe what she had seen and she independently verified what he looked like, where he appeared from, and where he disappeared to. We agreed that we had been sitting in the same position on the sofa when we each saw the guy, and we agreed that there was no alarm or malevolence about the guy, just general spookiness because we both felt that he didn't belong here.
Then we laughed and forgave each other for fighting, and continued on being best friends. This was the first and only time I ever saw a "ghost", and although I'm sure it was not his intention, I'm still grateful to him for his part in bringing my friend and I back together!