Met Leonard Nimoy in a hotel lobby. There was a scifi convention in town he was scheduled for and I was in town to do some work unrelated to said convention. I was unaware of said convention in fact... Anyway sat in the lobby and noticed an older gentleman sitting next to me waiting like myself for our rooms to be ready and I struck up a conversation realizing as he lowered the newspaper it was Mr. Spock himself. I didn't fanboy/nergasm out on him, but treated him like a human being and spoke to him at length about all sorts of things non-Star Trek related. Awesome experience and he was a really cool guy.
Another time, again surrounding a SciFi convention which I was actually attending, I ran into a bunch of guys in the coffee shop of the hotel lobby. They looked like skater punks (this was in late '98) and I immediately hit it off with them having a coffee and talking about random crap. At some point the conversation drifted to the GWAR show that was at the convention later that night and the guys at the table asked me if I was going. I said maybe if I could get tickets and they slid two backstage passes to me... I took them, thinking it was bogus but thanked them anyway and went out to the Con floor. Later that night I went to the GWAR show and surprisingly the passes were real. I went to the front of the mosh pit and GWAR came out, and I was looking around for the guys who gave me the passes when I noticed the bassist looked oddly familiar... Upon closer examination I realized the bassist and the band were the guys I was chilling with drinking coffee. I got pulled up on stage and put through the meat grinder, covered in fake blood and other fake body fluids... And got to hang out backstage after the show with GWAR, sharing a laugh at how they completely snuck up on me.
Pure awesome, and thanks to the GWAR guys for making my convention experience that much more awesome.
Also at the same convention I ran into Doyle from the Misfits... literally and I swear that dude is made out of stone cuz I felt like I ran into a wall. He picked me up with one hand and said "Watch where you're going kid" in his Brooklyn accent and walked off.
EDIT: I also have spoken with RA Salvatore many times online both in chat and through e-mail. The man is pretty cool and has some great views on the world, but we don't always agree. Still its nice we can talk civilly and be friendly (in an online sense) while holding diametrically opposed viewpoints on some things.