Did you know the entire line-up scene in The Usual Suspects was unscripted? How about Matt Damon's story about his brothers in Saving Private Ryan? Or even Bill Murray's Cinderella story in Caddyshack?
Here's a very cool video of the best unscripted scenes in Hollywood history. Our favourite's gotta be Sgt. Hartmann from Full Metal Jacket. The language is so NSFW, but you've got to marvel at that kind of inventiveness in cursing.