So here's a list of a dozen of the favorite classic space operas and monster films of all time. It's not meant to be a definitive list, nor does it include those which are strictly considered to be horror films; for that, see my Halloween list of Scary Movies.
A Trip to the Moon (1902)
Frenchman Georges Melies' 14-minute film based on Jules Verne's novel From The Earth to the Moon was the first science fiction film. He was the first to use stop-motion photography, the fade-in, the fade-out, and the dissolve. There are very few techniques used today that weren't employed in that film; we just do it a lot better now.
Metropolis (1926)
The first great sci-fi feature film was a silent masterpiece of German Expressionism by Fritz Lang (no relation), depicting a 21st century anti-utopian workers' hell. Several sites devoted to this film allow you to walk through the city, compare the different versions and soundtracks, and view the details of the reconstruction of the film. And here's a review by Roger Ebert.
Things to Come(1936)
Based on an H.G. Wells novel (with a screenplay by Wells) and directed by William Cameron Menzies, this film echoes many of the issues tackled by later futuristic stories.
When Worlds Collide (1951)
Involves a subject that was recently revived -- collisions or near-collisions with other heavenly bodies. The Laserdisc Review calls it "the original planetary disaster movie." The special effects were wonderful at the time, but some of them suffer in comparison to the current crop of asteroid collision films. It's still a classic.
The Day The Earth Stood Still (1951)
This breakthrough film set the tone for many sci-fi flicks which followed.
War of the Worlds (1953)
Another H.G. Wells' novel turned into a ground-breaking film in which tentacled Martians in deadly spaceships invade Earth but are destroyed by the smallest of our inhabitants. Well known because of the famous Orson Welles radio play. Read more about it on the War of the Worlds Web Site.
20,000 Leagues Under the Sea (1954)
James Mason is a great villain, and Kirk Douglas a swashbuckling hero in the cartoonish Disney version of Jules Verne's classic science-fiction story. It's a "whale of a tale."
Forbidden Planet (1956)
A sci-fi version of The Tempest that's sexy and scary, especially for 1956.
Invasion of the Body Snatchers (1956)
When I was a kid, this was the first horror/sci-fi film that I was forbidden to see (probably had something to do with the word "body" in the title). The 1978 remake wasn't bad, either.
The Time Machine (1960)
H.G. Wells once again. His 1895 novel is one of his greatest, and this George Pal-directed flick starring Rod Taylor brings it to life in memorable fashion. It won an Oscar for Best Special Effects.
2001: A Space Odyssey (1968)
Kubrick's film revolutionized the science fiction genre. It also "starred" one of the great villains in movie history, who wasn't human or even technically alive.
Close Encounters of the Third Kind (1977)
Made the same year as Star Wars, this film is distinguished by its humane, hopeful quality. It's probably true that only Spielberg could have made such a film a box office success.
There are lots of other Web sites dedicated primarily to older sci-fi movies. All are listed in our 
