I've met a bunch of famous people for various reasons:
Coincidence: I was waiting to pick up a friend at the airport, and Jimmy Carter got off at the adjacent gate. I shook his hand. One of my college buddies was Liz Jackson, daughter of Phil Jackson...never met him though.
Location: I live in the Dallas area, quite near the Cowboys' offices & practice fields in Valley Ranch, and there are celebs all over the place. The Metroplex is lousy with them- its a great place to live- I'd imagine that only cities like NYC, LA and Miami have more per sqare mile. Examples: The Stars (hockey team) used to hang out at my local Japanese restaraunt, ZZ Top used to go to my neighborhood Mexican restaraunt, I've seen Shawn Bradley (Mav's center) at IHOP, met Troy Aikman at Bennigans & Hooters, I've met Ken Hitchcock (Stars Coach) and Herschel Walker (Cowboys/Vikings RB) in grocery store lines, Clarence Gilyard (the black guy on Walker, Texas Ranger) went to my church, sat in a restaraunt with my back to Jerry Jones (Cowboys owner), and encountered Martina Navratalova and Richard Simmons at Nieman Marcus.
Right place & time: I had a buddy who interned at Kerrang! magazine, and when she returned to the USA, was supposed to be a freelancer for them, but she didn't have a car. Therefore, I drove her to concerts for which she wrote the reviews or interviewed bands, and got to see shows at cut rates and get backstage! I got to meet Pat Travers, LA Guns, Rick Emmit, Queensryche, King's X and many, many more...mostly bands you've never heard of, like Dirty Looks and Shark Island.
Then there was working SXSW. I sold T-shirts & CDs to people like Robert Plant and David Geffen. That just kind of warps your brain.
If I look around me, my Mom was friends with the Neville Brothers before they became famous musicians...and more than one of them was considering a life of crime.