Six Pool Rush
The Zerg have the fastest rush found in the game and on the smaller maps, this can be quite devastating to an opponent. Simply set the first four Drones to gathering Minerals, produce the 5th and 6th Drones as soon as you have enough minerals and command each to gathering minerals as soon as they are produced. While your Drones mine, send your Overlord to quickly find the enemy base and keep it there to observe what your enemy may be doing. Keep an eye on the Mineral amount indicator in the top right of the screen and once the mineral count rises above 200 Minerals or so, grab the last Drone to drop off its Mineral load and bring it out of the mining area and prepare to set down the Spawning Pool as soon as the Mineral counter tops 200. While the Spawning Pool builds, build one more Drone and send it to mine and continue to allow the other Drones to mine. As soon as the mineral counter tops 100, build another Overlord. By the time the Pool finishes, you should have over 150 minerals, enough to convert the 3 Larvae at your Hatchery into 3 pairs of Zerglings. Use these to quickly rush to the enemy base and target enemy workers one at a time (queue up the targets if possible by selecting all your Zerglings, right clicking on an enemy worker, then hold down Shift and continue to right click on more workers). This should effectively disrupt their mining operation and if your opponent fails to effectively counter with their workers, your Zerglings should be able to finish off the entire enemy base.

This strategy is quite limited in its usefulness though. The keys are finding the enemy quickly and hoping that you can hit them shortly after the Zerglings are produced. On larger maps like Killing Fields or Sherwood Forest, the time it takes to find the enemy and actually hit them with the rush would give your opponent more than enough time to build up defenses or even just enough workers to overwhelm your rush force, so if you do decide to use this tactic, limit it to 128x128 or smaller maps if you want to pull it off successfully.

Note that on very small maps like Blood Bath, you can even pull a Four Pool Rush by only mining with your inital 4 Drones and waiting til you have enough for the Spawning Pool and proceeding with the rush in the same way as the Six Pool Rush. This tactic provides an extremely quick rush but if it fails after the first two waves, consider yourself dead since you will not be able to produce quickly enough to press the advantage.

While both rushes have the potential to be devastating, an experienced player can defend against even these quick attacks in which case, they will most likely outmine and outproduce you in a short period of time. Weigh these options before electing to use these particular tactics.

Multiple Hatcheries
Typically, your initial Hatchery serves as both the source of all your initial units as well as being the point where incoming resources are processed. While one Hatchery provides sufficient larva for the early needs of a Zerg Brood, to truly produce the Zerg hordes, you'll want to build multiple Hatcheries at your main base solely for unit production. Having these extra Hatcheries will help alleviate the resource surplus you'll usually have at this stage of the game and also allow you to produce and replace units quickly between and during battles. Should your resources allow, also build extra Hatcheries at your expansions. In the event that your main base is overrun, the additional expansion Hatcheries will give you the extra Larvae you'll need to quickly rebuild.

Once you have multiple Hatcheries set up, use the hotkeys to speed up production. You can assign each Hatchery to a Ctrl group by selecting the desired Hatchery then holding down Ctrl and then the number you wish to assign it to (i.e. Ctrl - 1). Once assigned, pressing the number will reselect the Hatchery.

After a Hatchery has been selected, the option of Select Larva becomes available and by hitting the hotkey S, all the Larvae for that Hatchery will be selected.

After the Larvae are selected, all you need to do is hit the hotkey for the unit you want produced, and the selected Larvae will morph into that unit. The hotkeys for the units are as follows:

D - Drone
Z - Zergling
O - Overlord
H - Hydralisk
M - Mutalisk
S - Scourge
Q - Queen
U - Ultralisk
F - Defiler
 

Once you have learned these skills, use them in quick succession to produce your units at an incredible rate. For example - if you've assigned your main base Hatcheries to the numbers 1, 2 and 3, you can mass produce your units by pressing 1 - S - H, 2 - S - H, 3 - S - H and Hydralisks will be produced using all available Larvae at each Hatchery. If you wish to make any other units, just substitute the H in each combination with the hotkey of the unit you wish to produce. Using this method, you should be able to produce units even in the midst of a battle and by the time the battle has ended, you should have a fresh force awaiting command at your Hatcheries.

Unit/Ability Combinations
The special abilities of the Defiler and Queen provide a deadly complement for the main combat units of the Zerg Horde.

Zerglings with Dark Swarm/Consume
Use Dark Swarm over an enemy force or base and a group of Zerglings will be able to reign freely beneath the cloud. Produce masses of Zerglings to get the full benefit of the Dark Swarm and use multiple Dark Swarms to quickly cover an entire area. The Zerglings will be able to quickly move from target to target while remaining relatively protected from enemy fire. Also by keeping the Zerglings close at hand, Defilers can Consume them to rapidly recharge their energy stores so that they can continue to produce Dark Swarms for the Zergling attack force.

Mutalisks/Devourers with Ensnare
Use the Queen's Ensnare ability to slow down an enemy air force's movement. Should you be attacked by Cloaked Wraiths, Ghosts or Protoss forces under the protective cloaking of a nearby Arbiter, Ensnare will reveal the hidden units and even the odds. Swoop in quickly with your Mutalisks or Devourers and inflict damage to the enemy while they remain affected/revealed by the Ensnare. Devourers add to the Ensnare affect with their own acid spores further affecting the enemy forces' cooldown rate.

Ground Forces with Spawn Broodling
Use groups of Queens to quickly clear out the enemy's most powerful non-robotic ground units and before they have a chance to react, hit them quickly with your ground forces (Zerglings, Hydralisks, Ultralisks). Spawn Broodling is particularly useful against Terrans using Siege Tanks as the linchpin of their defensive formations.

Devourers/Mutalisks/Scourge with Plague/Ensnare
This combination can be deadly for enemy capital ships (i.e. Carriers or Battlecruisers). Use Plague first to begin the HP drain, then quickly follow up with a few Devourers to attach their acid spores to cripple the enemy's ability to counterattack. Before they can get away, use a quick Ensnare to slow them down and then once the effects have taken their toll, use Mutalisks and Scourge to finish the job. With practice, enemy capital ships can be taken down in a very short time span whether they are alone or in groups.

Nydus Canal
The precise origin of the Nydus Canal is unknown, and the exact process of its operation is also a mystery. Once the initial canal entrance/exit is created, a sister entrance/exit node can be opened at another site (as long as there is Creep to grow on). The Canal then enables Zerg ground units to travel from one end to the other at extremely high speeds regardless of intervening terrain. This enables multiple Hatcheries located across a large area to function as one unified nest, with Zerg warriors traveling from one combat zone to another quickly and efficiently.

With the ability to instantly travel from one location to another with Nydus Canals, the Zerg can quickly defend and invade often before an enemy can react. Typically, you will want to build all the entrance nodes at the main base and their sister nodes at each of your expansion colonies. This will allow quick travel from one base to another, at most with only one stop between locations. By building a solid network of Nydus Canals between your main base and all of your expansions, you can make sure that each thrives with a mobile force protecting them.

While one Nydus Canal at each area is usually sufficient, in some cases, you may wish to build backups at heavily attacked areas to ensure that your force will be able to arrive in full force. Should the enemy target one of the Nydus Canal and manage to destroy it, your force should have enough time to pour through the 2nd Canal to stop complete destruction of the colony. These redundant Canal systems can be the saving factor in a heated invasion.

The one drawback with building multiple Nydus Canals is that they all look alike and determining which node goes where can become quite confusing. To help prevent this type of situation, plan the construction and placement of each node so that you can distinguish where each goes. One suggestion is to build the Nydus Canals on your main base Creep to indicate which direction each transports to. For instance, if you wish to make a Nydus Canal that has an exit node at the top left resource area on the map, build the initial Nydus Canal at the top left part of your main base's Creep. Whether you use this convention or make one of your own, by building up a solid system for Nydus Canal construction, you'll be able avoid confusion in the middle of a critical situation and get your troops to where they need to be at any given moment.

The Nydus Canal can also be used as an invasion tool against other Zerg. While the inital entrance node of the Nydus Canal must be set down with one of your Drones, the sister exit node can be placed on the Creep of any Zerg Brood as long as you have direct sight of the location you wish to set the node down. This means that once you build the first node, use one of your flying units (Scourge, Queen, Mutalisk or Devourer) to take a look at the enemy's Creep and then produce the Nydus Canal's exit node at the enemy base. If there are any Sunken Colonies within range of the Nydus Canal, they will instantly attack, however, if you can find a remote position somewhere on the enemy Creep, the chance of a successful node being set up increases and once it finishes, you'll be able to send all your ground units through to invade the enemy from within their own base. Be sure to have a full attack force ready to go through before you set down the exit node, that way they can take advantage of the situation before the enemy finds the node.

Another use for Nydus Canal building on foreign Creep, is to use them to link with a teammate's/ally's Zerg base. By using these Canals with friendly forces, you'll be able to send reinforcements quickly should the need arise.