Leading Edge Games developed an Aliens RPG and a miniature game as well.

Scroll down a bit on this link:
http://www.waynesbooks.com/Aliens2300ADinspiration.html

There is a free online RPG.  Looks a bit rough around the edges but the guy put a lot of time into it.

http://www.serenadawn.com/AlienRPG-Contents.htm

If you decide to try and pickup the Leading Edge games on Ebay...good luck.  They almost always go for extremely high prices.

GamersRest[FNG] wrote:

And I personally suggest not trying to actually run the LE version.  The rules are a bit, how to say this, insane at times (if my memory serves me).

~jeff

I'll second that suggestion.  It's worth a read, just to say you've read it, but it's what you might call "a tad overcomplex."

The boardgame version by the same company is fairly decent, though.  There's one expansion for that, which is also worth tracking down.

The miniatures (again, same company) are fairly good, but it can be murder finding them.

Best bet is to track down a copy of the Colonial Marines Technical Manual (which is a pure fluff book unrelated to any game) and go from there.

Aww... *sniff sniff*  I registered here two years ago today!  And all of six posts.  

-Thornhammer