Visible crew/equipment: After Ripley fails to shut off the auto destruct sequence, she runs through a corridor filled with gas. As she exits from the gas right before the shot cuts away we see her pass by a full sized light. Beside this light is the clear arm and head of a lighting crew member. (This is acknowledged on the DVD commentary).
Visible crew/equipment: When Ripley is heading towards the Shuttle towards the end, just before she finds Jonesy, there is a POV shot where the camera is walking towards a ladder. If you look at the floor you can see the dolly that the camera moves on. (01:36:05)
Visible crew/equipment: When Kane is struggling on the table shortly before the alien rips through his chest, just before the first patch of blood appears, his T-shirt rides up and something strapped to his chest can be seen. This is some sort of pack containing fake blood that discharges onto his T-shirt. (Best viewed in slow motion.) (00:55:40)
Visible crew/equipment: Following the chest-burster scene, as the camera pans around the darkened corridors on the Nostromo just prior to Dallas ejecting Kane's body, what appears to be a boom mic is visible only for approximately two seconds to the left of the screen. (00:57:10)
Visible crew/equipment: When Ripley is in the escape pod and has the space suit on, she hits the button to open the door and blow the alien into space. In the shot just as the alien is getting sucked backwards, you can see the wire coming out of its back. (01:51:30)
Visible crew/equipment: As the Alien drops down behind Brett (before it kills him), you can see wires attached to the Alien's back.
Answer: It's another race of space alien that is also subject to infection by the Aliens. This particular one was infected and moved as far as it could from its race's known space and broadcast a warning before it died. The presence of eggs in the hold may indicate that it was a research ship. The race was never used in other Alien movies because it adds a new dynamic to the plotlines: two alien species, locked in mortal combat and neither particularly friendly with humans. In the Alien pseudo-prequel Prometheus we learn these beings are known as the Engineers and have interesting ties with both the aliens and humans as well.
Phoenix