Wait, aren't these things the enemy? Nope, not any more. At one point Sheppard picks one of these up and straps it to his arm (as you would). It can be used to grapple onto special organic nodes that appear on some walls or to grip onto enemies and drag them into the waiting, teeth lined jaws of the barnacle.
A rather odd weapon here. It fires mini-teleporters into the enemy to remove all but the largest of foes from the vicinity. However you can also turn it on yourself, shifting you to a location somewhere in the alien dimension of Xen to stock up on health and items.
Plucked from the arms of a slain alien Shock Trooper, this bio-weapon fires ten blasts of electricity in quick succession, recharging much like the alien gun Freeman got familiar with in Half-Life. Most impressive is the lovely effect of electricity shimmering over an enemy body before they either gib or retaliate.
This creature is quite simply the cutest weapon in the game. Don't let looks deceive you though. A strange aquatic looking creature, it gobbles some sort of biological ammunition and then spits it out with explosive results.