The Zerg have the ability to scout very early in the game with their airborne, yet slow, overlord. This is an advantage that the Zerg has over everyone early in the game, since they know where you are and the defenses you have setup. One mistake is to let the opponent see your overlord. This just tells almost as much information of where you are, making your scouting backfire. So try to keep overlords over plateaus and non-land areas, so any early land unit won't detect them.
Burrowing is also one of the best scouting abilities in the game. Combining this with zerglings, you get two spots on the map to keep an eye on for 50 minerals. This is invaluable when you are forced to pull any stray overlords back to the safety of your base, because of anti-air units as wraiths, corsairs and scourges. Since hiding in the mineral patches is almost invisible to detect, using zerglings to prevent a builiding construction is also very useful. Not only will the person have to remove the zergling, it slows him down, since he must get a detector.
Scourges and mutalisks make great scouts as well. Scourges especially are useful since they are two units in one, making it easy to reveal the map. Both can fly, and both are very fast, making it easy to scout the map for any expansions. It also helps to eliminate other scouting overlords as well.
Queens are the best unit the Zerg have for scouting, not only for their spell, but their speed and flight. Unfortunatley, parasite costs 75 to cast now, making it slightly harder to cast, and requiring the upgrade to cast 3 parasites. Also the Terrans have restoration making it harder to parasite any unit, as long as they see you with queens. Just try to parasite an expensive unit, since cheap units usually end up suiciding into your base. Parasites are very useful on island maps, where it's important to keep an eye on all the shuttles, and drop ships. Of course critters make create targets, especially ones who tend to wander into another base. It's even better on the new maps, which have flying critters, making scouting easier.
