Two of my favourite 50s sci-fi movies, both 'ITs'. It Came From Outer Space is an acknowledged classic, of course, with its literate Ray Bradbury script which introduced many themes later to become cliches: the desert setting, misunderstood monster and aliens masquerading as humans. First (I think) use of Theramine soundtrack, creepy monster-eye-view camera work and the most sophisticated 3D process then invented - so sophisticated it was never used again and it's impossible now to see it in 3D (the excellent documentary extra on the DVD convinces you just what a real loss this is).
It- The Terror etc is much, much better than its naff title would suggest. Indeed Ridley Scott mentioned it as an influence on Alien. Spaceship goes to Mars to find out what happened to an earlier expedition, only to find everyone dead except, suspiciously, the captain. He babbles on about a monster coming out of the dark to kill them one by one, but they think he murdered his crew in order to live off their rations till help arrives. Needless to say, said monster climbs on board before they take off and in the depths of space starts killing off this crew, too. 'It' is just a bloke in a rubber suit, but is largely kept in the shadows, fortunately. The film becomes increasingly claustrophobic as the unstoppable creature gets through first one bulkead and then another, advancing floor by floor and forcing the crew into the confined space of the nose-cone. Good stuff.