The first chapter of my WIP is largely drawn from a dream. I'm not the sort of person who pays much attention to dreams, except of a general barometer of my unconscious, but about two years ago, while I was making notes for the WIP, I had a dream in which I was trying to solve a missing persons case. I stood on a dock at the edge of a lake and knew that if I swam out to a raft in the middle of the lake I would be able to contact the ghost of the missing person and solve the mystery. So I dived in and swam but I as I climbed up on the raft the dream faded away and I woke up.
I couldn't think of the dream as anything more than a random mess, but the emotion it brought was so strong that I gave it to the protangonist of my WIP and used it to begin the novel. The novel is not a murder mystery, so instead of a missing person, she is trying to swim out to meet her dead mother, with whom she had a troubled relationship. I still think this makes a very strong opening to the book. Something about the idea is just solid and right. I just wish I knew who I had to thank for it.