Crawling, Slimy Things Terror-Bent on Destroying the World! This classic 1958 horror flick is fanboy-notable as it's somewhat based on the novel the Puppet Masters by Robert Heinlein. Although the producers never quite asked Mr. Heinlein for his permission, so he sued them and the case was settled out of court. Years later the Puppet Masters would go on to be made as a bad film in it's own right in 1994 (which thankfully Heinlein didn't live to see).
StarTrek fanboys should also have this film on their radar as Leonard Nimoy (that's Mr. Spock to the rest of you) plays the role of Professor Cole in the film. Directed by Bruno VeSota (who did quite a bit of character acting in the 60s) the plot of the film centers on crafty alien parasites who are set to take over the world by twisting the emotions of our women and turning our men into robots as seen in this trailer:
Poster found via the Internet Movie Poster Awards.