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Col Needham once watched Ridley Scott’s “Alien” 14 times in 14 days. In 1990, he watched 1,100 movies. That same year, he founded what would eventually become the Internet Movie Database, an invaluable research tool for movie geeks and the best friend of movie critics who, off the top of their heads, can’t remember all the titles of all the films in which Walter Brennan appeared in 1936.

As of last week, when IMDb.com celebrated its 15th anniversary, the site contained information on 471,378 film titles and nearly 1.2 million people, including stars, directors, key grips and best boys. Company officials point out that about 30 million unique users call up the site on their computer screens every month.

Any way you count it, that’s a lot of traffic, making IMDb one of the most visited sites on the Internet, ranked consistently in the top 50. And it all started because Needham couldn’t keep track of all the movies he saw.

“Movies have been a major part of my life all the way through,” Needham, 38, says over the phone from his home in Bristol, England. “I began to see so many movies, I would lose track of which movies I had already seen. So I began to create my own private database of films I had seen, who was in those films and who directed them, when I saw them ... so I could avoid renting the same film twice.”

Needham was soon sharing his information with a movie-discussion group, rec.arts.movies. Because there was no World Wide Web in those days, he would send his database to other movie buffs, who in turn would send theirs to him. Because the majority of his compatriots were male college students in the United States, Needham says with a laugh, most of what he received were lists of actresses and the movies in which they starred. It would take awhile before their databases gave equal time to both sexes.

Today, it’s rare that a movie question can’t be answered with a visit to IMDb.com. Yes, mistakes do pop up occasionally, and because so many people consult the site, erroneous information can spread quickly.

“We are not infallible,” says Keith Simanton, IMDb’s managing editor. “With almost 2 million people listed on the site, it’s very possible to get things wrong.”

But when mistakes are pointed out, officials of the site take pains to correct them.

The IMDb’s value as a reference source for popular movies is undisputed. If there’s a criticism of the site, it’s that more esoteric films, along with older films and lesser-known filmmakers, are sometimes given short shrift.

“The biographical and other ‘background’ information on non-celebrities tends to be scanty at best, and non-commercial films get distinctly second-class treatment,” says David Sterritt, chairman of the National Society of Film Critics.

Still, even a tough audience like Sterritt admits, “It’s a flexible enough database to be of use to an enormously wide range of people.”

True enough. It’s probably settled more than a few bar bets in its time too.

(By the way, Walter Brennan appeared in six films in 1936, including “Come and Get It,” for which he won the first best supporting actor Oscar. You can look it up on the IMDb.)