PropsGuild used to have all of the various incarnations of the aliens and Ellen Ripley characters, by SixPacWolf (now known as Mesh_Magick) and others, but after it became a subscription site the characters were removed.

sunscape had a power loader suit in Free Stuff, but the link is broken. Questor had one at his site, also, but I'm not sure if it was ever posted for download.

JAFO has a morphing alien egg in Free Stuff. That link, at least, still works.

davo has a free Mechalien character (robotic alien) at BeyondBent. While you're there, check the store for his Nightmare Creatures collection, which has a number of disturbing Alien-esque monstrosities, including a facehugger.

For ship interiors, check out lalverson's offerings in Free Stuff, or visit his website, Alverson Fantasy Art Studio.

And be certain to visit the Movie Mesh site, which has some great models of the ships and weapons from the Alien saga in .3ds format, with texture maps.