Every now and then one stumbles upon something that is just bizarre -- like discovering that the girl who played Door in Neverwhere also was the blacksmith girl in A Knight's Tale. Can't put into words why that is so weird...you'll just have to take my word for it.
I love IMDB, though, for that very reason -- looking up old movies and discovering what happen to the actors, that they went on to do other things you might have heard of and never realized it was them. I would say 2 out of 3 times that John and I watch a movie, I come upstairs afterward and look up the actors to see what else they've done. Like discovering that the father from Sound of Music went on to play a Klingon in Undiscovered Country. Or that the oldest son in the same musical went on to play the 70s TV Spiderman. Or that one of the younger daughters went on to play Penny Robinson on Lost In Space. (Or that she appeared as a reporter in the Lost In Space movie.)
Eerie and weird.
So, what are your favorite examples of an actor going on to do another project and most of us probably don't realize it's the same person?