I have a few:
On my 50th birthday, my husband and I saw "A Steady Rain." Afterwards, the stage door was CRAZY crowded but my husband, (bless his well-projected voice and long arms stretching out through the crowd) managed to get me both Hugh Jackman and Daniel Craig's autographs on our Playbills (this was before they stopped doing stage door autographs in order to sell them for charity.)
Years ago, Tim Curry played William Hogarth in "The Art of Success" off-Broadway, and I was right up front. He did a rough sketch of a woman character during the play and then threw it on the floor nearly right in front of me. As soon as the play was over, both I and a man a few seats away reached for the sketch at the same time, and our eyes met. He smiled, and let me take it. Chivalry is not dead!

I framed the sketch and have it to this day.
Last but not least, during the Broadway Flea last year I bought several Polaroids taken of Neil Patrick Harris when he was trying on different dorky glasses for a costume designer when he was about to play Mark in "Rent". Okay, that was the L.A.production not NY, but I bought it at Broadway Flea so it counts...right?