From concept to screen there is a painstaking amount of work in between, and very rarely does the final product look exactly like what was initially conceptualized. Such is the case with director Ridley Scott's 1979 sci-fi classic Alien. Here is a bunch of concept art and rare images io9 came across, some of which appears to be screenshots from the rare Alien/Aliens laserdisc.
Dan O'Bannon's and Ron Cobb's original Concept Art:
The most drastically different the from the now iconic look of the Aliens, is creator and screenwriter Dan O'Bannon's original concept art for the creature -- an E.T. sized, half-crab, half-dinosaur looking creature with a face similar to the Predator, that would of course be ultimately rejected. Below that is Ron Cobb's design before H.R. Giger was brought on...
Alien with Human Skull:
The front part of the alien costume's head is made from a cast of a real human skull, which is completely covered in the film. In these next two pics, you'll see on set pics of the original alien's articulated jaw, and a translucent version of the alien's cowl.
H.R. Giger's Concept Art:
Giger's designs for the creatures would ended up being the ones used in the film, but here is some early concepts of the Alien and his original facehugger concept art from back in 1978.
To see more of H.R. Giger's artwork visit his website.
Ridley Scott's Storyboards:
Ridley Scott drew his own storyboards for the film, here a couple.
James Cameron's Paintings/Concept Art:
Here are some paintings/concepts James Cameron did for the 1986 sequel, Aliens, which we all know he co-wrote and directed. They include concepts of; the alien planet, alien egg sacs, and of the alien queen.
Just a little bonus, Cameron rejected this concept Syd Mead did for the heavy loader.
For set concepts that Cobb did, head over to io9.