Alien pulls from quite a few different films that fall within the "Dark House" formula. IT! The Terror From Beyond Space (Fox almost got sued for plagiarism due to similarities in the stories), The Thing, Planet of Vampires, The Thing From Another World, Texas Chainsaw Massacre (though this was more of a directorial influence for dealing with horror). There is even a claim that another influence for the film was a story by Agatha Christie.
Look at nearly any Dark House style film (Sci-Fi or otherwise) from the 1950's through the 1960's and you're likely to find elements that seem strikingly similar to ALIEN.
ALIEN was not an original story, it actually adheres to it's parent formula quite closely, but what makes it stand out was it's treatment of the characters, its presentation of the film's environment, its quality of script, and the quality of production. If any of these had been sub-par ALIEN would most likely have been regarded as a B-Movie, and overlooked about as quickly as say "Saturn 3."
I remember seeing that in the Alien Legacy documentary. It was a combination of minds using inspiration to develop something that will endure for years.
Well, since Dan O'Bannon made "Dark Star", and even said that he had originally started writing "Alien" as a serious remake of the comedy "Dark Star", one should only hope that "Dark Star" reminds you of "Alien".
Just a slight off topic word on the director. Edward L. Kahn also directed 2 films that were an influence on Romero's Night Of The Living Dead/. That film was the 1958 classic Invisible Invaders. If any of you have seen this one the influence is apparent. The second one was called Creature With The Atom Brain. See em' and you'll see what I mean
There was a 1958 film that was a possible influence upon the first Alien film. The film was called IT! The Terror From Beyond Space and it was directed by a guy called Edward L. Kahn. The plot was basically the same-a creature who stowed aboard a ship to another planet, who then kills the crew off one by one while the ship's commander has to explain why it happened. (keep in mind Ripley's inquest by a jury of her peers at the beginning of Alien 2) Anyhow, while the monster isn't as spectacular, this little B-movie has been said by some critics to have been an inspiration for the original Alien film. See it and I think some of you might agree. Good stuff.