I've always had a copy of Halliwell's Film Guide close at hand for looking up details of old films, but it's not got much of an index, so you can't ask whether Robert De Niro and Cybill Shepherd appeared together in any films besides Taxi Driver (they didn't).
So I bought Microsoft's Cinemania as soon as I had a computer with a CD drive. It was fun searching for a while, but it's information was often incomplete, and of course, it was out-of-date as soon as it was pressed.
But now my prayers have been answered. The IMDb answers every question, about everything. And they even have a UK version.
Wondering whether Zuzu was a one film child star, or whether the actress appeared in any other films? Go to the IMDb and look it up (or type Karolyn Grimes in the search box). Had an argument about who played the little girl in Léon? Go to the IMDb and be friends again (it was Natalie Portman).
And if you're just browsing, drop in for the daily poll or the quizzes.
John