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."