Years ago, I seem to remember a facehugger prop that came in a plexiglass case. The 'Hugger was painted and assembled, it was not a kit. The cool thing about it was it had a rubber piece on its underside that you put in your mouth to hold it to your face. Think I saw it in Starlog. I used to have the Halcyon kit, it was great.
I converted one of the vinyl kits into a mask. The arms are flexible enough. I just tied several of them together with elastic material. I cut a couple of eyeholes and put a mesh over the hole. After painting the mesh is not that easy to see.
The tails doesn't always bends as nicely but with some care it can look convincing around your neck.
(scratches head...) I bought one on ebay a few years back... but I'm not sure what it was made from. its not very flexible although it looks great.. I did a decent painting job on it although I might try to touch it up.
the one in the pexiglas case was manufactured by don post studios in the early 80's. it was a limited edition when it was made, expensive and very rare.
halcyon came out with the model kits, which could be adapted into a wearable piece (thew vinyl was softer than that of a normal kit). this was recently re-released by a japanese company.
collegeville/imagineering came out with a mask where the fingers were connected with elastic, the tail velcroed around your neck (there were hidden eyeholes between the knuckles on the body) and it featured a pair of bladders and a straw that you could blow into to make the gonads pulsate. it came with a t shirt with the comapny name from aliens on it.
distortions unlimited and morris costumes produced a foam filled latex version. toward the end of the run there were severe quality control issues. (both collegeville and distortions/morris produced chestburster t-shirts.. with a rubber burster sticking out of the torn shirt.) this is almost certainly the one d_jedi describes (and jedi, theres a very good chance you bought it from my brother and i.)
all of the above are now discontinued. morris/distortions also produced the mask shown in the MiM website ad.
currently the license to manufacture "aliens" related halloween products (life size props often fall under that catagorization) is held by rubies costume company.
well, the one I bought came out of England. had Royal mail postage and everything its a good kit. I wish I had pics of how it looked before I built it. I'll try to get it from my mother's basement tomorrow and take some pics to post on my photobucket account so you can see what you think about it. It might have been vinyl, but it was kind of hard. Not styrene hard.. but still.. meh, I have NO experience with vinyl kits so what do I know