For those that have seen Prometheus, Screen Rant has a very good story on the connecting tissue between the Alien and Prometheus movies. It is an interesting and good read. I still haven't seen Prometheus yet, because I've been really busy these past days. But, I read it anyway despite the major stories.
Side note: Event Horizon is currently on TV right now, and it could have easily fit into the Alien universe. (EH is a good movie often overlooked.)