I really hate to say this, but in Multiplayer, the zerg are near unstoppable. I discovered this by accident but: Infested Terrans make you invincible.
You can start building Infestors pretty early on, and they have the ability to spawn an Infested Terran which now shoots an infested Gauss Rifle. The energy cost to spawn these is so low, each Infestor can pop about 7-8 of them. Now if you spend enough resources into building up an army of infestors (A group of about 7-12 of them), you can spawn an absurd amount of Infested Terrans.
Now, Infested Terrans die after a short time. But the damage that a group of them can do being spam spawned off 7+ Infestors is ABSURD. They can bring down a group of Thor and Battlecruisers like nothing. This makes them viable defense options. Let your infestors build up energy, and if you get attacked, just spam spawn Infested Terrans and I assure you, the enemy attack will fail.
Then we come to the attack option. See, infestors can move while burrowed.
...
MOVE WHILE BURROWED.
This means you take your group of infestors, burrow, and charge right on into the enemy base undetected. Granted, if the enemy is smart, they will have enough detectors. But the point i'm making is that you make enough infestors, charge in and start spam spawning Infested Terran, and you will cause such a crazy amount of damage to their base, if not fully destroy it.
Note: Infestors and Infested Terrans are relatively weak on their own. But sweet buttery jeezus, if you have them in Mass, nothing short of a nuke is going to stop you.