Zerglings As Observers:

One way to help yourself out as any of the 3 races is to keep a constant flow of resources. A fast flow is best wanted. To do so, you have to expand and make sure the enemy doesn't know about your actions. If he knows, he'll most likely attempt to destroy your new base, canceling your ideas. To make sure that when you do expand the enemy doesn't know, early on, put Zerglings at possible expansion points and burrow them. This way they act like Observers and keep that area visible to you. Now you know if the enemy will know of your actions in moving there. Another way to use Zerglings as Observers is to place them in choke points which the enemy must cross when making a move by ground. This way you know of his actions and can prepare accordingly. Zerglings can't detect cloaked units, so you can't make them like Observers in that way, but I'm sure there are many more ways as to use the Zerglings for instant recon.

Instant Recon With Parasite:

Now, many Zerg players use this strategy on their enemy, but you can do other things to get recon with parasite as well. Probably everyone knows the old tactic done by Zerg players in which they take a Queen, go over to the enemy base and parasite a Probe/Drone/SCV of the other race. This used to be a great strategy before Brood War came out. Now all a Terran has to do is use Restoration and get rid of the Parasite. If you're lucky, you may be sly enough to get the Parasite on one of his units without him ever knowing. But that's rare, and even if it's a different race, if he has a Terran ally, he'll more than likely get the ally to Restore him. I've found a better way to get recon with Parasite is to go around the map and Parasite every critter you can find. The best one to Parasite is the Kakaru (the bird in the Twilight world), because this bird actually flies almost over the entire map throughout the game. Hardly no one ever clicks on these things, and therefore will not know your friendly critter is giving you a look at the enemy base. If you put the Parasite on one of the more stationary critters, you can use them to alert you of any moves the enemy makes. If you get one in front of your base, you'll see them moving to you, and if you get one in front of the enemy base, you can see them moving to expand or attack you and have even more notice to prepare. Parasite is basically a one-purpose ability, and doesn't have a variety of uses. But the one use it does have, I suggest using to its full extent.

The Defiler and Dark Swarm:

One of the fewest Zerg units I see used on Battle.net is the Defiler. This is a unit I expected to see used a LOT. I mean, it has Plague and Dark Swarm. Even better, he has the Consume ability which gives him a boost on his energy recharge rate. Here I won't talk about Plague, but I'll mostly talk about the Defiler's innate ability, Dark Swarm. This spell is by far one of the most least used abilities. This spell has the capability of literally saving a Zerg's base. Think about it; if your base is getting heavily hit by either ground or air and your Spore/Sunken colonies are getting ripped apart too easily, what better way to keep them intact longer than you throw a Dark Swarm over them? Just have a bunch of Zerglings standing nearby to give the Defiler more energy as needed, and you can cast this spell indefinitely. This helps keep your colonies (as well as your units themselves) more difficult to inflict damage upon, and thus giving you the advantage. Zerg need this advantage when having a large assault on their base, cause Zerg depend on shear numbers. The Zerg units and structures are way too easily killed, and therefore would greatly benefit from this tactic. Dark Swarm can also be used on offense. While attacking an enemy base, if you get caught up in a choke point, rather than letting his Tanks or Reavers, or whatever easy pick off your units one by one, throw down a few Dark Swarms on your front lines and let them work their way through. Be careful not to use it on your enemy's units or structures, because then it will help them and hurt you.