If you're talking about chance encounters then I've never had that before. I've seen Dexter Fletcher while queuing up outside the now dead London Astoria to see Apocalyptica last Dec but I didn't say anything to him. As for conventions and signing sessions yep I have met a few people.
Examples of those who are memorable because I had a quick chat with them or they simply had presence are:
* Amber Benson @ Collectormania 14. She was super nice and appreciated my compliment on her new hairdo XD
* Dean Andrews @ Collectormania 14. Very chatty to all the fans who came to see him at said event. Took his time to get to know people. All round friendly guy so different to the Ray Carling character.
* Michael Rosenbaum @ Collectormania 14
* James Kyson Lee @ Collectormania 12. Still the coolest Heroes actor I've met by far. Had more things to talk with him :)
* Gary Barlow @ a signing session where he was promoting his autobiography in Waterstones, Oxford Circus branch.
* Nightwish @ a signing session in March 2008 in Zavvi, Tottenham Court Road branch. I got the band's autographs quickly because the security were rushing the fans to get their items signed.