OK so you might have heard a little about ESCs (Electronic Speed Controllers).  The essential elements to understand about speed controllers for brushless motors is that they are generally a device for producing a pseudo alternating current that is applied in a timed fashion to the three coils inside the brushless motor.  We say pseudo alternating current because the current is really a switched DC voltage which ends up "looking " like an alternating signal to the motor.

Well OK about the alternating current but what makes the motor speed up or slow down?  This is usually done by the signal from the receiver, but with Quads, we have the flight control board in the middle to send the signal to all the motors.  Depending on the Quad you choose to build, you will either be using standard off the shelf ESCs or the I2C type.

Standard ESCs are also known as PWM (Pulse Width Modulated) types and apart from their varying load current capabilities, they are distinguished by programmability and update rate capacity.  Which is why you cannot use any old ESC when building a Quad.  In fact, the update rate capacity is very important since your Quad controller could be updating your ESCs up to 400 times per second (400Hz) or more depending on the flight controller board you choose.

The other type of speed controller used is the I2C (actually pronounced "I squared C" and stands for: - Inter-Integrated Circuit - ) which are designed to be used in high speed update situations .  In other words, you can use update rates in the 1000s of Hertz.  The other interesting fact about I2C ESCs is that you can actually take a PWM ESC and convert it to an I2C ESC with some careful track cutting and soldering.  For more information click here.

Some ESCs  (Electronic Speed Controllers):

Turnigy Plush 25A		The Turnigy Plush ESC is the one that I would recommend as it is IMHO one of the most reliable. Yes, there are many cheaper ESCs, and while you can  get an ESC for less that will probably do the job, why take the risk. For the sake of the difference in price it makes sense to go for this one and get peace of mind in the bargain.
HobbyKing SS		There is essentially nothing wrong with the Hobbyking SS Series 25-30A ESC (card programmable).  This ESC is a great performer and also cheap.
TowerPro w25A		The TowerPro w25A also does the job very well and some people have successfully converted these to I2C.