As far as bassists go:
Jason Newsted - backstage at one of the Seattle shows for the Black Album tour. Jason is a really nice guy. Rapped with him for probably 20 minutes about stuff.
Tony Levin - after a Tony Levin band show a few years ago. Got to chat for a few minutes, mostly about playing with Funk Fingers and diffirent techniques to use.
Eddie Jackson (Queensryche) - I was really good friends with his little sister; we went to school together. I hung out with him so many times I can't even remember...
Stu Cook (Creedence Clearwater Revival) - My band opened for Creedence Clearwater Re-Visisted in Las Vegas last November. Got to meet Stu and chat with him when they were done with soundcheck as we were preparing to sound check. He hung out and listened to us for a bit. Nice guy.
Bryan Beller (Mike Keneally Band, Steve Vai, etc) - I've known Beller now for years, extremely good bassist and overly humble about his greatness.
Krist Novoselic (Nirvana) - He's tall. Really, really tall. I met him backstage at the Metallica show as well.
Tom Fowler (Frank Zappa) - Met him before a Banned From Utopia show.. basically Zappa's band after Frank died.
Jeff Ahmet (Pearl Jam) - complete and utter jerk. I met him at a Seattle Seahawks game. He was with (then drummer) Dave Abris...(however you spell it). Dave was a fantastically pleasent person and I talked with him at great length about a lot of stuff. REALLY cool guy. Jeff seemed annoyed by my presence and was generally a complete a%#hole.
Christian McBride - at a Guitar Center of all places... I actually was noodling around on a bass and he listened to me for a while and finally started talking to me. I had no idea who he was when I was playing, as I didn't know what he looked like... I don't think I would have been playing "Donna Lee" and "Teen Town" in front of him if I had knew who he was...
non-bassists or whole bands...
Wierd Al Yankovic and his band - They love a band that I used to be in (Travis Shredd & the Good 'Ol Homeboys). I have hung out backstage with them on numerous occasions and took them out drinking after one of their shows. Al and drummer Bermuda actually came to a TS show in Seattle one time; that was cool stuff.
James Hetfield - obviously met him the same time I met Jason. James was cool.
Pee Wee Ellis & Fred Wesley (James Brown) - Met these two after the Pee Wee Ellis band (feat. Fred Wesley) got done playing. Talk about meeting some funk legends, that was really cool.
Wynton Marsalis - got to meet him (and play trumpet for him... good god) after a Lincoln Center Jazz Orchestra show in Seattle. He's a great cat man.
The Mike Keneally band - met everyone in the band and talked with them after a show in Seattle. Talk about good people.

I'd met Mike several times before that and talked with him at great length about all kinds of stuff.
Non-musicians, but still famous
Wayne Gretzkey (the Great One), Keifer Sutherland (actor), Cam Neely (hockey player), Tom Arnold (actor), the Hansen Brothers (from Slapshot), Russ and Jeff Courtnall (hockey players).