I met Joe Straczynski at San Diego Comicon 1995. My brains promptly turned to mush and I babbled something incoherent. My wife, bless her, was much more articulate.
I've spoken briefly with Neil Gaiman a couple of times, but not so as he'd remember me.
Nicholas Cage was at the same comic store I was once, but I didn't notice him until someone pointed him out as he was leaving.
I got to chat with Bill Mumy once at a con. He was extremely pleasant. Forrest Ackerman came by while we were waiting in line to see Bill, but he was mostly interested in flirting with our friend in Esperanto.
I met Jackie Chan at a book signing and shook his hand. His bodyguard, Ken Lo, was also there (the tall kickboxing guy at the end of
Drunken Master 2), and he seemed very appreciative when we told him how much we liked him in that movie.
Harlan Ellison came to speak once at the University of Michigan when I was an undergrad, and I got to talk to him a little when he signed a book for me. He seemed to be in a good mood that day, and was very pleasant.
I met J. K. Rowling at a book signing. She looked completely exhausted and a little confused when she noticed that my wife and I were about 20 years older than the majority of the people there. She didn't say much, and I didn't blame her one bit.
That's pretty much all I can think of off the top of my head, unless you count the occasional Jon Bon Jovi sightings at our local ice cream place when I was a kid in New Jersey.
-- Pazu