I have "seen" quite a lot of famous people, as my best friend used to live in Los Angeles and worked in fashion. I stood next to Drew Barrymore and her then boyfriend "what's-his-face" from The Strokes in a tiny little bar in West Hollywood once. She was a lot smaller than I thought she would be.
But the most famous person I have actually had a conversation with, must be Ryan Qwanten (before True Blood). He was really polite, sweet and quite shy.
Oh, and I drunkenly "jammed" with an American singer/song writer, Pete Yorn at his home. He's not very well known outside of the States, so I doubt he counts.