I’ve seen Kurt Cobain, Clarence Clemons of the E Street Band, Jerry Garcia of the Grateful Dead, and Prince. A pretty fair collection, I think. I had tix to see Johnny Rotten and the Sex Pistols but they cancelled. I think Johnny was too f’ed up.
[Have]
George Carlin and John Denver.
Separate performances. 
“Take me home, dirty words…”
I saw Queen, with Freddie Mercury. And I saw David Bowie 
I don’t see a lot of live music, but James Brown and Doug Bennett (of Doug and the Slugs) both died not long after I saw them perform.
(Note to self: update this thread if Keith Richards ever dies.)
Screaming Jay Hawkins
Perhaps in anticipation of his mention in this thread, he appeared on stage in a coffin. It was a great show.
Harry Chapin, Muddy Waters, Frank Zappa, Mark Sandman, Alvin Lee, Rodney Dangerfield, original Lynyrd Skynyrd, original Canned Heat, Cub Coda, Lena Horne…
The barefoot diva from Cape Verde, Cesária Évora. I saw one of her last performances, then met her after the show.
I saw Anson Funderburg & The Rockets featuring Sam Myers many times, including Sam’s final tour. Sam chain smoked from grade school on, battled cancer, but kept on smoking and singing to the end.
I saw and hung out with Koko Taylor during her final tour. Same with Pinetop Perkins. I shared a quart of vanilla ice cream with Pinetop (long story).
Saw Link Wray play amazing surf guitar shortly before he passed.
I met Son Seals, saw him perform, and escorted him to his hotel afterwards just a year before he died. He was touring because he was broke, had lost a leg to diabetes, and was basically playing for insulin. He had lost a bunch of siblings and had a few house fires where he lost all his treasured possessions. Representing the blues.
BB King was another great one I saw throughout his career, including toward the end when he could barely walk.
Johnny Winters. Helped him travel the fifty or so yards from his tour bus to the stage at the Pittsburgh Blues Festival back when it was held at Iron City Brewing. I carried a folding chair and Mr Winters sat down to catch his breath every ten yards or so.
Jack Bruce, Jimi Hendrix, John Bonham, John Entwistle, Keith Moon, Keith Emerson, Greg Lake, Chris Squire, Clarence Clemons, Roy Bittan, Linda McCartney, Chuck Berry, B.B. King.
That’s all I can think of.
One that I saw was Ernest Tubb. My grandmother’s favorite singer, it was 1979 and I was 7. He was playing at the local “opry house” that also happened to once have been the school my grandmother and her siblings went to in the 1920s. So I was along with my mother, grandmother, and a few of my grandmother’s sisters. My grandmother was tired from standing in line and we decided to go sit on the back porch for a while (which my grandmother was familiar with from her childhood.) After a while, a band member steps out for a cigarette and invites us inside out of the cold. So we hung around backstage with the band before the concert. Ernest was busy, but did take the time to come speak to us and shake hands.
[Moderating]
I fixed the typo in the title. It was making my brain hurt until I figured it out.
[Not moderating]
Tommy Makem, multiple times (but most of you have probably never heard of him). He used to always come to the Cleveland Irish Cultural Festival every year.
Thanks
Went to a lecture by Stephen Hawking in the mid-'90s. Buckminster Fuller as well, probably 1982 or so.
If we’re restricting this to musicians, I went with a friend to see Morphine at a street fair in Cambridge (MA). Their lead singer, Mark Sandman, died about two weeks later.
I saw Stevie Ray Vaughan at a small club in Detroit called St. Andrew’s Hall. He had recently left Bowie’s band to go solo, and I didn’t really know who he was at the time but my friend told me he was great. “Sweet Little Thing” was getting a lot of airplay, so I knew him from that song.
It was a great show. We were right up at the stage. Very intimate setting. At one point some idiot shook up a beer bottle and sprayed it all over Stevie and a 50’s Telecaster he was playing from a large collection of vintage guitars he had on stage. A roadie jumped off the stage and ran after the guy.
Bowie was actually playing in town that same night, earlier at Cobo Hall, and there was a rumor going around that Bowie would show up at the end of Stevie’s set and do a couple songs with him, but sadly that never happened.
Afterward we waited out in the alley by his tour bus to say hey to him and maybe get an autograph or something, but he didn’t show and we got tired of waiting. Probably for the best- if we surprised him in a dark alley in Detroit we might have gotten beat up by his roadies. Would have made for a better story now, though 
When I was in university in the nineties, I met and talked to Majel Barrett, who had been the wife of Gene Roddenberry, when I working the Chicago Comicon at what was then known as “The Rosemont Horizon”, I believe. For those of you unfamiliar with her, she played “Nurse Chapel” on the original series and the mother of “Counselor Troy” on “Star Trek: The Next Generation”.
She was so darned nice friendly and nice to me, and she gave me a signed picture of herself. It was a great experience.
John Lee Hooker, BB King, Jerry, Chris Squire, EL&P, Sam Kinison, maybe a couple more I can’t think of…
Oh! Tom Petty and John Denver, too. And that fruity magician Doug Henning.
Bowie
Stevie Ray
John Entwistle
George Carlin
That’s all I can come up with.
I think we can safely call the scope of this thread “performers”, not just “musicians”.
Buddy Hacket
Ray Charles
Chris Cornell (Soundgarden)
Oh, yeah. Forgot about Chris. Or maybe I just refuse to accept he’s gone.