While hanging around the stage doors at Broadway I've managed to see Alan Rickman (Snape, for those too lazy to Google) and Arthur Darvill (Rory, from Doctor Who).
I discretely followed Rickman down a block or two in order to get a better look at him, then stopped when I realized I was being creepy. Only got a glance at his profile, but I heard him speak and it was definitely him.
As for Darvill, I was able to have a brief interaction with him and even took his picture. The man is even sexier in person.