The Nydus Network

                 Back in SC1, the Nydus canal was frequently used to join up the Zerg's expansions with his main, to quickly deploy defilers/lings to save expansions.  With the introduction of the new Starcraft 2 structure, the Nydus Network, what once was a purely defensive structure, is now a critical offensive building, that too few players exploit.

The Nydus Network immediately after Lair

                 The Nydus Network costs costs 150 minerals, 200 gas, and has a build time of 50 seconds: just a few seconds faster than a roach.  The second your lair is finished, a Nydus Network could be built for a bit less than the cost of two hydralisks.  With its ability to pop ANYWHERE you have vision, its almost imperative to get it ASAP. Getting the Nydus network into play as soon as possible has many benefits.

Many players often over commit their Nydus attack.  They would load every unit in their network and do a major "doom drop" in their opponent's main.  This has disastrous consequences (especially vs P.) because it leaves the zerg player open to attack.  It is useful to get into the mindset that the Nydus worm is a great harass tool, and not meant for an all out means to kill your opponent.

                 The nydus worm costs 100 minerals, 100 gas (the cost of a hydra) and comes out in about 20 seconds.  If you lose your nydus worm, you DO NOT lose your network.  This makes dropping nydus worms frequently, a very good idea.  Often, I would put lings with whatever I plan on harassing with, and pop a nydus worm in my opponent's base.  Immediately, I would run my lings straight into the worker line, and try and do as much damage. 

                 Often, my opponent's retreating army would end up slowed by the bulk of my harass, which allows my lings to do sufficient damage.  Generally, bearing in mind the amount of damage I've done, I would take this opportunity to take up an expansion, and continue to try and out macro my opponent.

                  Not every base is "nydus-able", due to a small main base, or an extra observant opponent.  In this case, there is nothing wrong with an overlord dropping creep outside your opponent's base, but in an area he would most likely not scout (in between empty bases, perhaps).  Going for "backstabs", where your opponent's main army is containing you would benefit greatly, allowing you to move out and position your main army.  And in this case, your opponent main not generally be able to find the nydus worm you used, especially if you placed it well.

Kulas Ravine

                 A strategy I found very useful on Kulas Ravine: Your opponent's closest third base would be the one between his main and natural, enclosed by rocks.  Almost every game, as a harass, I would have my spotter overlord up there drop creep and get a Nydus worm out.  Destroying the rocks is easy, as my opponent would usually be macroing or putting his attention on his economy.  The opponent hears no sounds, and unless they actually click on the rocks, would not see the health drop.  Once I break through, a group of lings straight to the worker line, and the rest of the harass out to the choke to meet the retreating army.  At this point, I almost always take an expansion (sometimes two, depending on how ahead I already was).

"Nydus Mining", quite the skill to have.

              Of course, by about late game, the Nydus ends up quite useless.  However, many people have discovered a use for "nydus worm mining".  This is how it works:

              Once you're sure you no longer can use your Nydus network to attack, drop some creep at an untouched expansion, hopefully behind the mineral line.  Lay your nydus worm when you can.  Set the rally point for your nydus worm to a mineral patch, then hold shift and right click your nydus worm.  What this does, is when a unit comes out, it will mine from the mineral line, and then return back into the worm.

              For your Nydus Network (hopefully, you've planned ahead and placed this building next to a hatchery, which would make things so much faster) have the rally point clicked on a nearby hatchery, and then hold shift and right click the Nydus Network. 

              Units that come out of the Nydus Network will go to the hatchery, and then back into the Network.  I generally hotkey my Network "0" because it "starts" the process, and my Worm "9".  I fill my Network with 8-10 drones, and "9D", which deploys my drones out at the mineral patch. I wait roughly 15-20 seconds, depending how long it takes all the drones to mine and how close your worm was.  Then I press "0D", which unloads all my drones at my hatchery, and very quickly sends them back into the network.  If your Nydus Network is right next to your hatchery, within a few seconds you can continue, "9D", wait 15-20 s, and then "0D" again.

               It does take a bit of practice, but if you can place your worm well, then a scouting worker might not stumble upon it. (Especially if your opponent scouts only part of the main, and leaves when they don't see creep).