Well I've seen tons, but I've only actually met a couple

I met Hilary Duff in a restaurant in Vancouver once while I was up there for a long weekend, and the other was that bald guy from Lost (I forget his name), met him at a brewery in Hood River while I was there for a couple weeks windsurfing.

Both very friendly, and I actually keep in touch with one of them

Bruce Campbell, Rosario Dawson, Funkmaster Flex

Shaun Wright-Phillips and John Terry at some football thing at school years ago.

Charles Barkley (NBA)
Shaq (NBA)
Shaun Alexander (NFL)
Webster Slaughter (NFL)
The Allstate Guy (TV)
John Peterson (PGA)

Mostly football players. I won't include all of the LSU/NFL players that I had classes with despite meeting them (and people I just said hi too). Also know a few people in bands, but I dunno if Hillsong is popular enough to count.

Mike Bibby when he actually meant something on the Sacramento Kings.

Are we talking on the street or at a signing?

If it's the latter, then I've met quite a few.

Michael Crichton
Harold Bloom
Toni Morrison
Michael Ian Black -- got to interview him :)
Kevin Smith
Jason Mewes
Eve Myles
Euros Lyn
Mark Hamill
Carrie Fisher
Eddie McClintock
Tricia Helfer
Christopher Judge
Bruce Campbell
Gillian Anderson
David Duchovny
William Shatner
Gene Wilder

I'm too much of a loser to do so.

I almost met Will Smith...does that count?

I was an extra on the first series of Teachers and met Andrew Lincoln and an unknown James Cordon. I also shook the hand of the lead singer of Reel Big Fish after a gig.

Princess Anne, when I was like 13 or 14 I think. My school was opening a new science wing and she opened it, for some reason I got picked out of 4 from the entire school to represent the students.