David Hasselhof (does he count?) -- in the beach parking lot in Seal Beach, CA in late November/early December 1983. They were filming an episode of Knight Rider ("White-Line Warriors" from season 3), and my nextdoor neighbor talked me into driving down there. Nothing was happening, but there was no security, so we found his trailer and knocked, and he was watching Monday Night Football with a couple guys. He took Polaroids with the two of us, but I don't have mine anymore.
Vinny Appice -- at a drum clinic in 1985-86ish. He signed the inner sleeve of my Sacred Heart album.
All of Queensryche and Metallica -- at a meet & greet for winners of a radio contest in 1989. All of QR, except Eddie for some reason, signed the inner sleeve of my Warning album. All of Metallica (including Jason) signed the outer sleeve of my Master of Puppets album.
All of Fates Warning -- in the parking lot of a tiny club in Anaheim, CA in 1989 or 1990. All of them signed a mock of the Perfect Symmetry album (the sort of thing that used to get put up on the wall in record stores).
All of Queensryche -- at the taping of MTV unplugged. Four of the five (including Eddie, but I'm not sure who didn't sign) signed a set list that I snagged.
All of Dream Theater (except KM) -- in the parking lot of a tiny club in Westminster, CA in 1992. All except KM signed my I&W CD booklet.
Alan White and Tony Kaye -- in the parking lot of the Palace in Hollywood in 1993 after a Dream Theater concert.
All of Dream Theater (including KM) -- in a different parking lot in 1993. Filled in the missing KM signature in my I&W booklet.
Mike and Marlene Portnoy -- at Foundations Forum in (I think) 1993.
Billy Greer (Kansas), maybe someone else from Kansas, Steve Howe and Alan White -- backstage at a co-headlining show in 2000. I think I got them to sign something, but I can't remember what.
Macy Gray -- at a mediation in 2002 in which my firm represented her.
Slash -- in an elevator at my office (although we didn't say anything to each other, so I'm not sure if it counts as "meeting"). Not sure exactly when.
Rob Halford -- at a mediation in 2013 and at dinner the night before.
All of Yes -- at a meet & greet in Vegas in 2013.
I'm sure there are a few others that I'm not remembering at the moment.