I don't meet really, really famous people very often, but I meet a lot of "cult famous" people on a regular basis just as part of work. Not going to name names of those people, wouldn't be professional, especially as I despise most of these people on a personal level (with only a few exceptions).
As for really, really famous - probably the most well-known person I had a personal conversation with was Tom Morello. Although I didn't meet him in the flesh, I had a very long phone chat with Necro once, that was certainly enlightening.