Whoa. That is pretty damned cool. 
Dang, should have mentioned Jimmy Stewart. He spoke at my high school, wearing his uniform. Must have been a talk about joining the military.
I saw Stevie Ray Vaughn, B.B. King, and Joe Cocker do an encore together a few weeks before Stevie died. B.B. and Stevie played their guitars, and Joe grimaced along.
I saw Tom Petty when his current hit was “Jammin’ Me”. I saw Billy Preston with Ringo’s All Starr Band. Clarence Clemmons was there, too.
Forgot some people in plays I saw.
Jack Klugman
Tony Randall
Yul Brynner
Mickey Rooney
Anne Miller
And from when I saw television shows taped.
Johnny Carson
Ed McMahon
Dan Rowan
Dick Martin
Dinah Shore
Jimmy Dean
I’ve seen Ray Davies twice, but he’s very much still alive.
Ninjad.
I’ve laid eyes on many celebrities no longer alive (actors, etc) but I’ll stick to musicians:
Lennon, Harrison (Beatles, 1965)
Sonny Bono (with Cher, ‘66)
Zal Yanovsky (Lovin’ Spoonful, '66)
Morrison, Manzarek (Doors, several times 60’s)
Doug Ingle, Erik Brahn, Lee Dorman (Iron Butterfly, '68)
John Bonham (Led Zep '69)
Keith Moon, John Entwhistle (Who, '73)
Bob Brozman
Gregg Allman, Butch Trucks (Allman BB, '72)
Virgil Fox
Frank Zappa
Keith Emerson, Greg Lake (ELP '74, '77)
Captain Beefheart ('76)
Dan Hicks
Jimi Hendrix '69
Johnny Winter '69
Vinnie Paul. That reminds me, both him and Dimebag Darrell.
This might take a while.
Jerry Garcia
Vince Welnick
Brent Mydland
Miles Davis
Dizzy Gillespie
Ella Fitzgerald
George Carlin
Garry Shandling
John Lee Hooker
B.B. King
Roy Buchanan
Albert Collins
Johnny Winter
Stevie Ray Vaughan
Matt “Guitar” Murphy
Willie Dixon
Koko Taylor
Etta James
Probably a whole bunch more that aren’t coming to mind just yet.
Davey Jones
Leslie Gore
Fats Domino
Probably more, but those are the ones I can think of off the top of my head. When I was a teenager, our local oldies station sponsored a lot of concerts with 60s acts and I saw a lot of those.
1990’s
At a Hollywood Premier of the documentary The Haunted World of Edward Wood Jr:
Forrest J Ackerman
Dolores Fuller
Rev. Lyn Lemon (Baptist pastor whose church financed Ed Wood’s Plan 9)
At the L.A. University Cathedral (the original United Artists Theatre & now a Performing Arts Center):
Dr. Gene Scott
Since 2000 at local music festivals:
Tommy Ramone (Uncle Monk)
Johnny Winters (Edgar’s brother)
I’m sure there are more.
Only ones I can think of:
Scott Weiland (a couple times with STP)
Saw the Deftones with Chi Cheng as the bassist before his car accident.
Saw Dick Gregory speak when I was in college.
Oops, spelled his name wrong. Ray Davis. He played for P-Funk. The make up a major part of my list. Sorry for the confusion!
Jerry Garcia
Tito Puente
Several of the above-mentioned blues musicians
But the saddest was Ofra Haza.
Mary, of PP&M; Sammy Davis, Jr.; Jonah Jones; Ed McMahon (worked with him on/off stage once); Henry Mancini (on stage, in person); Liberace (worked with him in a studio); Carl Ballantine (worked with his daughter); Dave Brubeck, Joe Morello, Paul Desmond (on/off stage, San Francisco). Probably some more, but I can’t think of them right now.
Gord Downie. Sigh.
Richard Wright
Chris Squire
John Wetton
Jon Entwistle
That’s all I can think of off the top of my head.
Lemmy - Motorhead
Phil “Philthy Animal” Taylor - Motorhead
“Fast” Eddie Clark - Saw him with Fastway after he left Motorhead
Benjamin Orr - Cars
Ronnie Montrose
Jon Lord - Deep Purple
Rick Wright - Pink Floyd
“Lonesome” Dave Peverett - Foghat
Hughie Thomasson - Outlaws
Ronnie James Dio
I saw Johnny Cash a few times, most memorably at the Cubby Bear in Chicago, a small, very intimate venue. I stood right up front and got to shake his hand after the show.
Also saw Tom Petty & the Heartbreakers in '95 with my then-girlfriend, now wife. We like very different kinds of music, but Petty is the rare artist that we both love, so that was a pretty special show.
I also wanted to add more Temps and a few Tops but didn’t want to search for names. Which, apparently, I should have originally done because look how dumb I look for not doing research first.
John Lee Hooker
BB King
Jerry Garcia
Ella Fitzgerald
David Bowie