Here's my pretty stupid idea, but I'm bored at work, and think it'd be funny: Pellet Gun, causes enemy unit to go berzerk, from annoyance or something. (just read the "before posting in weapon concepts" thread, which should probably be above this one, so people read it first... so if this is currently impossible, sorry.)
I'm new here, so I'm not sure what's been discussed, and I havn't quite gotten the game to work, but it all seems really cool. My computer at work only has MSpaint, but I'll try to donate some ideas for now. How about some sort of hairy bug/spiderlike alien? the Arachtoid or something