Sectoid (Bureau)

A slave race (who's iterations will be tied to XCOM's (and Earth's), future for years to come) under the control of the Outsiders, these Grey aliens are numberous, fairly weak, and easy targets. But, they seem to be under the more direct command of a higher intelligence, which drives them to surprsingly clever tactics, including cover fire and flanking.

Having no additional abilites, and only wielding pistols (starting with Laser pistols, and eventually upgrading to Plasma varieties), they focus more on superior numbers to swarm XCOM agents. Having small, almost childlike bodies, they need to nearly climb over their own cover to be able to fire at Carter and his team. Plus having massive heads, it's fairly easy to dispatch them with small-arms fire, or a single headshot from nearly any weapon.