It! The Terror From Beyond Space (1958)
This has been on my watch list for quite a while now. Seeing as John Carpenter selected it as his second film after The Thing From Another World while guest programming for TCM, I decided it was as good a time as any to watch it. What a great film! It's all about space crew as they try to survive an attack from an alien stowaway. It also served as the inspiration for Alien. =P