Besides the obvious fact that Valve’s character-heavy first-person adventure game Half-Life 2 and Ridley Scott’s psychological haunted house space flick Alien both have plots centered around ordinary humans pitted against less-than-friendly extraterrestrial adversaries, they both have several parallels that lead us to believe that Half-Life‘s developers may have taken some ques from the 1979 film. The clearest example of this comes from one of the many enemies Gordon Freeman fights over the course of Half-Life 2. Although the primary enemies are the ever-present, terrorizing forces of the Combine (after a hostile alien force attacks Earth and Breen organized humanity’s surrender, several people either volunteered or were impressed into service to maintain a police state), Freeman also faces “headcrabs” and their victims. These creatures, originating from the same rift in space-time that the Combine used to reach Earth after the incident at Black Mesa in the first Half-Life, attack living organisms and attach themselves to their faces (hence the “head” in headcrab). Once attached, the victim usually dies and is made into a monstrous vessel that attacks others.
This is very similar to the namesake of Ridley Scott’s Alien. When a deep-space mining team is interrupted on their journey home to investigate a distress signal on an uncharted planet, they encounter an organism that, once free from its egg, attaches itself to one of their crew members. So, like a headcrab, this creature is parasitic. However, the “facehugger” from Scott’s film uses a tube from its center to lay an embryo inside the host’s body. After a short time of incubation, the alien bursts from the victim’s rib cage, killing him or her. This is by no means suggesting that Valve’s designers shamelessly ripped off H.R. Geiger’s (creature designer for Alien) idea, but the similarity is peculiar nonetheless.
For reference (headcrab’s top, facehugger’s bottom):