Celebrities before they were famous
It's funny sometimes when you go back and watch older movies and you realize that guy with the bit part or the kid is actually (insert celebrity name).
Like Russ from National Lampoon's Christmas Vacation now plays Leonard on Big Bang Theory (Wow, look how young he was there!)
Or certain movies that now have an all-star cast... but they didn't at the time they were filmed. Like The Outsiders, for instance. I bet most people can name evey member of the cast now, but they were relatively small potatoes then.
Of course, the other aspect of going back and watching older movies makes you realize how old these actors are now. Kinda weird.