I'm cleaning up some old answers that might have been accurate at the time of writing but are completely incorrect now, so the updated answer:
In ZvT there are a few common game plans i have observed recently.
Muta/Ling/Bling into Broodlords
The classic, but recently falling out of favour. You open with Zergling/Baneling, get Mutalisks when the Lair finishes, harass the opponent until he dies and if he doesn't, you get Broodlords to finish him off. If he moves out too early for Broodlords, you build as many Banelings as possible and roll over his army with your hopefully superior economy.
This is very volatile since you need perfect Mutalisk control and if you lose some to a lucky Thor shot, the opponent can just move out and kill you straight away. Also, the current trend for heavily upgrades Marines allows the Terran to keep only a small number of Marines in the base while he's still safe. The high gas cost of Mutalisks also makes teching very hard, but if you keep upgrading mutalisks and manage to transition into Broodlords, they will spawn with good upgrades, which can give you a significant advantage.
Ling/Infestor into Broodlords
My favourite. You basically build only Drones and Zerglings while using all the gas for double Upgrades and a fast tech to some Infestors. As soon as the Infestation Pit is done and you research the Infestor Energy upgrades, you start the Spire and Hive at the same time and as soon as both finish (they have the same build time), you morph the Greater Spire and get Broodlords. After that it's all about positioning and Fungal Growth.
This Strategy needs very good map awareness and perfect Injects to always have the exact amount of Zerglings that you need to be safe. Banelings can become neccessary against Marine heavy compositions, but they delay your Broodlords, so think twice before getting them.
Ling/Infestor into Ultralisks
The basic opening is like Ling/Infestor above, except you get the Ultralisk Cavern instead of Spire/Greater Spire.
While Ultralisks were considered useless for quite a long time they are recently used a lot more and with a lot more success. The key is not massing Ultralisks but getting a few (~8) and then using them to tank Damage while a big number of Zerglings kills everything.
There are several key points to make Ultralisks work:
- You have to land good Fungals so the opponent can't kite them.
- Not too many Ultralisks so they don't block each other.
- Control each Ultralisk seperately to make sure they don't get stuck on buildings or in chokes.
Stephano played this for a long time with great success, before moving on to the more successful Ling/Infestor/Broodlord strategy.
Basically in ZvP you only need two options:
Roach/Mutalisk into Broodlords
Your earlygame Units are Zerglings and Roaches, the exact composition depends on the opponents composition. On this basis, tech to Lair and Spire, get Mutalisks and harass him until he gets forced into a desperation attack or rolls over and dies. Keep the Colossus count as low as possible but don't lose Mutalisks.
In the mean time you get Air upgrades, Infestors, Hive and Greater Spire followed by Broodlords and some Infestors, you then move out and kill him or defend his desperation attack and then move out to kill him.
This strategy is like the similar Ling/Bling/Muta ZvT Strategy very volatile. Blink Stalkers make Harass very hard and if he opens with some sort of non-cheese Stargate, don't even try it. Since the Patch that added the Phoenix Range upgrade, Phoenixes counter Mutalisks very easily and you won't be cost efficient unless he screws up his Phoenix micro.
Roach into Infestor/Broodlord
As a more stable strategy, you open with a lot more Roaches instead of Mutalisks, which leaves you with more gas that you can use for Upgrades, Infestors and faster Broodlords.
If he goes Stargate play, either get some Hydralisks (max 3 per Phoenix) and kill him with your Roach army or get a Spire for some Corruptors. You might want to get a Spire for Corruptors if you scout or suspect Colossi to counter those, you can later morph it into the Greater Spire for Broodlords. After that it comes down to Fungal on his Blink Stalkers and keeping them away from Broodlords.
Nope, they don't get used. Immortals rip Ultralisks to Shreds and while theoretically Fungal Growth can allow Ultralisks to connect with Blink Stalkers, i've never seen anyone do that successfully.
The (in my opinion) most volatile Matchup still hasn't really developed a solid lategame, it usually starts with Roaches and ends with Roaches (or with 2 banelings in your mineral line), so it's hard to write down a solid game plan, but i'll try anyways.
Ling/Bling into Roach/Infestor/Broodlord
From the early Zergling/Baneling wars, you transition into the common Roach midgame. From there, you get a few infestors out and try to survive with equal bases and equal unit count until you get some Broodlords out. As with all non-Mutalisk Broodlord strategies, you should start Spire at the same time as your Hive and then morph into Greater Spire as soon as both finish.
If the opponent scouts your Spire and the Hive he will most likely get a Spire for Corruptors, which means you need a similar amount of Corruptors together with your Broodlords. However, he might just decide to get lots of Roaches instead and kill you straight up since you sank a lot of gas into Tech that he now has in Units. You can stop that by adding a lot of Zerglings to your Roaches or counter attacking, but this is still the most dangerous moment in the game.
You then engage straight up and try to fungal everything that might shoot your broodlords (corruptors/hydralisks).
Ling/Bling/Roach into Ling/Roach/Infestor/Ultralisks
As with the Broodlord strategy you open up into a Roach/Infestor midgame. From there you slowly transition into Hive and Ultralisks.
You can get Melee Upgrades early on instead of Ranged to make better use of the Ultralisks in the late game but it's a risk in the Roach midgame. You then engage his Army and Fungal his Roaches in as big clusters as you can so you make the most use of Ultralisk Splash.
With the very Infestor focussed metagame at the moment, it's not a good idea to go Mutalisks if the Opponent has Infestors ready. 3 Infestors can kill all your expensive Mutalisks if you get caught in a bad spot and after that he can just move out and kill you.
It is possible to make Mutalisks work, but in my opinion it's not worth the risk.
It depends on what the enemy does. Mutalisks are a good solution for
the midgame and perfect for harassment and map control, but they die
quickly to hydralisks. Against a mainly Hydralisk army a group of
ultralisks and banelings (don't forget banelings in lategame) work
very well. He can either kite the banelings and get AoE damage from
the ultralisks or kill the ultralisks and get obliterated by
Anyways, try not to let it come to the lategame, contain your enemy in
the midgame with a big amount of mutalisks so you just get the
superior economy and can just throw wave after wave of units into his