Sparky
Shameless plug to the designer of Sparky 1 ... For information about Sparky 2.0 go here and buy his "sparky brushed" here: http://www.hobbiesfly.com/TauLabs-Brushed-Sparky-2-0.html .
This is a cheap flight controller designed by Peabody124 for Tau Labs. It is designed to be small and easy to assemble, while being highly capable for acrobatic flight, altitude hold, FPV, RTH or PH. It is open hardware and licensed as CC-BY-SA. For more details on the design and development see this blog post.
BE WARNED: Due to the open hardware nature of this project, numerous clones are available and they are not all equals. There have been a number of reports of boards from Ready to fly quads (also known as Witespy) having bad regulators and barometers. They fly but are unable to perform well with altitude hold or navigation. If you are looking for a cheap clone, you'd be much better of having a look at AliExpress and buy from a seller with good feedback.
Version 1.1 replaces the 4 pin rcvr header with a 3 pin header and a solder bridge to select the voltage. This allows using a standard 3 pin servo cable without modification.
Here is a small video that illustrates how to perform the dRonin upgrade on an "out of the box" sparky flightcontroller.
Here is the version 1.0 schematic and the version 1.1 schematic as pdfs.