Too many to name, far more than I can begin to remember. Honestly I can't think of when there wasn't at least a celeb or two when I've seen something.
I've bumped into Sondheim well over a dozen times through the years. He was sitting directly behind me the first time I saw Elaine Stritch At Liberty at The Public (I had to fight the urge to turn around and see his reaction to some of her stories about him). I saw him twice in one week once -- at the off-Broadway play "Blackbird" in this tiny theatre that sat 40 or so (he had to step over me to get to his seat) and then two days later at the final performance of Gypsy. He probably thought I was stalking him.
I've sat next to Sir Derek Jacobi, Michael Musto (more times than I can count), Hailey Mills, Rosie, Harvey Fierstein, Kevin Bacon and Kyra Sedgwick, Matthew Broderick (at Wonder of the World starring Sarah Jessica Parker), a row away from Eric Stoltz, Jason Alexander, Mike Nichols and Diane Sawyer, Ian McKellan, Tim Rice at Spamalot (he loved "The Song That Goes Like This"), Barbara Cook at a John Kelly show at the Joyce ....... this is boring. There are dozens and dozens of others, but I can't remember them all (frankly it'd be easier to name the celebs I haven't seen or met or hung out with over the years than the ones I have -- and I NEVER do stage door). I don't get star struck at all, so just seeing a celeb barely registers with me usually. If you go to a lot of theatre and shows in NYC, you ALWAYS see famous people (especially late in previews near opening night). It's no big deal.
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
[http://margochanning.broadwayworld.com/]
"The Devil Be Hittin' Me" -- Whitney