On Broadway, Mendes added Sally, Cliff, Fraulein Schneider, and Heirr Schultz walking up the stairs on the side of the stage as it was happening. Very effective.
Some more recent productions that I don't think were mentioned yet:
In the Heights-Blackout. Say what you will about the show, I don't love it myself, but that finale was pretty great)
Grey Gardens- Will You? Absolutely haunting
Urinetown- Act 1 Finale. Hysterical and a cliff hanger.
Caroline, or Change-Roosevelt Petrucious Coleslaw. Saw it live with Anika Noni Rose. Pure magic.
Tommy-Pinball Wizard. Classic song spun on its head.
Smile-Until Tomorrow Night. Howard Ashman knew how to make a number build.
Nine-The Bells of St. Sebastian. Once again, absolutely haunting.
Kiss of the Spiderwoman-Gimme Love. Fun, catchy song that gets turned on its head, as well.
"Sing the words, Patti!!!!" Stephen Sondheim to Patti LuPone.