If you're the type who lays awake at night obsessing over the name of one of Kevin Bacon's co-stars, or if you catch an old movie on TV and are haunted by the question of whether that really was a teen-aged Betty Grable in the chorus line, The International Movie Database will make your life much easier, guaranteed. This website has tons of detail on movies--synopsis, cast, crew--and it's cross referenced to make finding information on related queries simple.
Newer on the scene is the International Broadway Database. It offers information on current shows, casts, crews, understudies, music, and the theaters themselves. There is also information on Broadway history with detailed info on shows of the past, their various productions, and casts.