So far I'm aware of only 3 attempts to translate the Aliens (plus Predator) universe into a pen & paper RPG source: Leading Edge Games Aliens RPG, Aliens: Fuzion, and Project Dust-off.
I looked up the Leading Edge game, and I'm betting it must be some kind of collector's item, because it's impossible to find. This is the one I'm most curious about. It's a unique RPG where you play as a colonial marine (of course it has nothing to do with Predator,) and it's based on an old rules system called Phoenix. I saw a copy on eBay for $100 once. What was it like? Did anyone actually play it? Sporadic comments on the internet say that it was neat as a piece of fan service, but as an actual functional RPG, it sucked quite a bit.
http://www.pen-paper.net/rpgdb.php?op=showbook&bookid=3800Aliens: Fuzion is still available. This thing was put on the web around 2001, and as a source of information about the Aliens universe (including speculation,) it's pretty extensive. FUZION itself is a decent rules system, and it's free. However, the Aliens supplement seems disjointed, and the whole file is written and formatted like a text dump. Overall, you can't just download this thing and consider it usable; plus, I have no clue if the author is still around. It hasn't been updated in nearly five years.
http://www.serenadawn.com/AlienRPG-Contents.htmThe last one I saw is called Project Dustoff, which is a web-based Colonial Marines RPG. The authors promised a stand-alone RPG source guide back in 2004, but it's been put on hold indefinitely (presumably due to real-life issues,) and the (incomplete) .zip file is a 404. Project Dustoff doesn't look like it will see the light of day.
http://www.wolvesau.net/Aliens/Are there any other major RPGs to add here? I wonder if Fox will try and license AVP to d20 if the next movie is successful. (Since d20 seems to be in a state of transition to a new version, I doubt it.)