I've only mentioned the ones that I've had more interaction with than just staring at them from across the room (or across the stage barrier) ...even though the prompt is asking for who I've "seen", haha. There are probably a few more but I can't think of them at the moment.

Nathan Fillion
His brother was the Vice Principle in my elementary school (and my French teacher in grade 6). He came to the school one day and visited during our French class. (This was back in his Two Guys, a Girl, and a Pizza Place days)

Michelle Beaudoin
Her mother was the librarian in my elementary school. She was hanging out in the library one day. I saw her and was a little surprised because I did enjoy watching Sabrina the Teenage Witch (she was Jenny in the show) back then, haha. ...I miss her mom (Mrs. Beaudoin was such a nice lady! Best librarian ever! <3). =P

David Usher
At one of his concerts. He's such a cool, down-to-earth kind of guy. =)

James Kyson Lee
Met him at a comic convention. He's such a friendly guy!

the guys from The Envy (band)
Met them a number of times at some of their concerts.

Nathan and Dan from Madina Lake (band)
Signing at Warped Tour 2009. I saw the entire band perform, but I was only able to talk to these two, so the other guys don't count. :P

the guys from The Academy Is... (band)
At Warped Tour 2008, and then William Beckett once again at one of their concerts.

Alex and Jack from All Time Low (band)
At Warped Tour 2008, and Jack once again at another concert.

the guys from We The Kings (band)
At Warped Tour 2008, and Hunter once again at Warped Tour 2009.

the guys from Mayday Parade (band)
At Warped Tour 2008. And then my friend and I also got to see Derek and Jake perform a private acoustic set (there were around 10 of us there). Good times. :D

the guys from Carolina Liar (band)
At one of their concerts.

Ryan from Between the Trees (band)
At one of their concerts.

Nathan and Mike K. from The Midway State (band)
At a couple of their concerts.

the guys from Radioforhelp (band)
At a couple of their concerts and Warped Tour 2008.