OK, I know there are a lot of people posting suggestions but I think I would like to add a couple.
First of all, for the humans a gun to fire nets to trap aliens, the smaller the alien the longer it holds them still, or the other way around. The flip side to this a projectile vomit which is sticky for an advanged dretch. This goo would only last for about the same sort of length as the net and could just restrict the movement of the human rather than stop them cold (depends on how much goo was sprayed) and will affect bsuits as well as unarmoured humans.
Another suggestion is mines, have you ever wondered what to do with all those extra build points? What about mines, each mine can only be used once, will kill, or severly injure an opponent and can be detected only by the side who put them there.