I was flying to Atlanta one time (probably 2002ish) and hit some really rough air. I think it must have caught the pilots off guard or something, because usually there's some sort of warning (flight attendants take your seats, etc). There was none of that. All of a sudden I'm damn near airborne and I have to stretch to catch my laptop that has bounced off the tray in front of me. It was a real chore putting my computer away without bashing my head into the seat in front of me. There was probably a half hour or so of pretty aggressive elevation changes and bouncing around.
Nothing like what others have described, but bad enough that pretty much everyone on the plane was looking around nervously.