Overpool (vs. Protoss)
When using the no-gas variation, Overpool is nearly equivalent to the 9 Pool build. The major differences are the Spawning Pool timing and around 24 Minerals. The same information regarding the 9 Pool applies to this one.
This is an aggressive yet safe opening against Protoss. In the commonly seen Protoss Forge Fast Expansion, the Protoss usually sends a scout after building the first Pylon that reaches your base in time to stall or keep you from getting your expansion up. Overpool prevents this probe harassment by getting faster Zerglings which can kill the scouting Probe which in turn allows you to expand without delay.
Also, after scouting a 12 Hatch, Protoss players tend to get his Nexus up before any Photon Cannons, which makes you and him to be even in bases, which is a clear disadvantage for Zerg. This may even allow the Protoss to get his Gateway up before any Cannons, which will speed up the arrival of his first Corsair. This build forces the Protoss to build two Cannons before attempting to get his Nexus or risk a certain run-by harassment.
The build orders are presented in descending order of common usage. The first one is thus the most often used.
This variation is one of the most economic version of this build, where the first few Zerglings are mainly for preventing any cheese or early Gates, forcing the Protoss to build a Forge and Cannons before his Nexus, and kill off the scout/blocking Pylons. If the Protoss did fast expand, then the Zerg will take two bases after the initial Zerglings.
Alternative Economic Variation
This aims to get the expansion up a little faster, but still lets the Zerg manage to get up 6 slightly slower early Zerglings then the build/opening mentioned above. Zerg can still force Protoss to go Forge first, and can still if scouted, depending on map, attack a late forge and still pressure a Nexus first/12 Nexus build.
If you want to do more than simply threaten your Protoss opponent and want to do a very aggressive attack with Zerglings, the following more specific build which is a Zergling rush of sorts can been used. It is designed to have 12 Speed-upgraded Zerglings for an attack at the Protoss expansion. If Protoss only builds two cannons then you can easily destroy them and most likely win the game, and if he has more you can attempt a run-by.
The economic variation of this build can transition into any build that the 9 Pool build can transition into. See that list here.
The other two variations of this build are more similar to the 9 Pool Speed build, thus look here for possible transitions.
This build is particularly strong on maps with a wide choke that forces the Protoss to block with more Probes than he would like to. Some examples are: