Infestor (above)

Who am I? A primary support unit. These guys are your best friend in a battle ground. They've got no direct attack but assist other units with Dark Swarm, a kind of smoke screen and Infestation which targets other races' structures causing them to create infested Marines. The Infestor can also move while buried meaning it can get into enemies bases undetected.

Most likely to: Give you a nasty rash.
 
Swarm Guardian

Who am I? Morphed form the Mutalisk, this is the Zerg's daddy air unit and looks like a colossal flying millipede. Get a few of these out in force and they can wreak havoc with their long-range air to ground attack. Also spits out broodlings onto the ground for extra win-power.

Most likely to: Come to the rescue when you're taking a beating.

Corruptor (above)

Who am I? The dedicated air-to-air unit with a twist. This guy likes to infect enemy ships with a poison that turns their ship into a Zerg gun turret that shoots back at its own army. As ruthlessly contagious as a virulent bout of flu, these dudes can disable entire airfleets with ease.

Most likely to: Really irritate any one who gets on the wrong end of them.

Overseer

Who am I? An upgraded version of the Overlord who serve as airborne detector units. They've also got the ability to see cloaked and burrowed enemies. You need to keep building them for some reason or a voice barks "BUILD MORE OVERLORDS!" at you. So you do.

Most likely to: Sit in the background being boring.