About 5 years ago I was meeting a cousin of mine at the Los Angeles airport and happened to be there at the same time as Miley Cyrus was being surrounded by a mob of fans while trying to get into a car to drive off from the airport. The group was of about 400 people, and that's about as close as I've gotten in real life to a celebrity.
I've also talked with a few astronomers through email or on the internet who are somewhat famous, but likely not people who any of you have heard of. Mike Brown, Maik Meyer, Timothy Spahr, etc.