Know Your Zerg - Spotlight on Zerglings

Zerglings have it tough - they are sacrificed as scouts, area-of-effect (AOE) weapons slaughter them, they're always the first into combat, and they never get the bed to themselves.

So... what are they good for?  Zerglings' main purpose switches from core army in early game, to scouts and runbys late game.

Are Surprisingly Good At:

• Counterattacks:  Even 12 Zerglings into someone's unguarded expansion might cause their main army to turn around to defend.   While 12 Zerglings shouldn't turn the tide of the battle for you, if they kill 12 enemy workers, the next battle will be far easier.
• Killing Roaches and Stalkers, early game.

Zergling Tricks

• Get Burrow.  Burrow Zerglings all over the map, preferably under enemy expansion locations and where you DON'T have creep, but need vision.
• Put a Zergling outside the enemy base.  If he moves out, you'll know it.  All it costs you is 25 minerals, and is easily and quickly replaced.

Words of Caution

• Although it may LOOK like you have a lot of Zerglings, they are so cheap that numbers are often deceiving.
• Do not send Zerglings against walls, into AOE, or through chokes.  Do not send them against large masses of marines, except as a means of preventing them from kiting your banelings
• Keep Zerglings away from enemy banelings.  You are almost better not engaging at all than engaging enemy forces anywhere near banelings.  If you must, send 1 zergling at a time into banelings to reduce their numbers 1 for 1.  It is possible to kill over 16 Zerglings with 1 Baneling.

Know Your Zerg - Spotlight on Mutalisks

Here is some key information on Mutalisks.  All information relevant as of patch 1.3:

Are Surprisingly Good Against:

• Void Rays:  equal numbers of mutas will kill a nearly equal numbers of Void Rays.  Void Rays cannot escape Mutas, and are more expensive.

 Are Countered By:

• They cannot outrun Phoenixes - stay and fight unless you can find nearby corruptors or other anti-air to save them
• Are shredded by most anti-air, but especially low damage, high-rate attacks such as Marines or Hydras

Best Purposes

• Should be constantly on the move, preferably harassing something:
• Workers
• Enemy Reinforcements
• Slow enemy units (Siege tanks, Immortals, Banelings

Upgrades

• Attack upgrades are better for them, since you will generally be using them to attack things that can't fight back, or that need to die fast, such as turrets.
• If you intend to make the "big Muta ball", armor upgrades are actually quite good.  Still, mutas should not linger when they are being target-fired by enemy units.

The Magic Box

• The magic box technique , conceived by Team Liquid, is a positioning tactic where you mutas are spread out and not overlapping.  This allows them to take on even scary units like a Thor or two without taking massive splash damage.
• To magic box, give the mutas a move command to a safe area, and then wait until they all arrive at the spot - they will naturally spread out.
• When attacking with your magic box, do not target-fire units.  If you do, the mutas will lose their magic box and clump up.  Instead, use a move command and park your boxed mutas over your target, using the Stop command when your mutas are in position.  Mutas have a bounce-attack and will hit what you want them to most of the time, since their range is so short.
• Watch a video tutorial on the magic box and its benefits