The basic problem is this: my controllers and peripherals are 3.3V or 5V, but the motors need as much voltage as I can throw at them.
The board is split into several sections.
The first part is the power intake. It's a screw terminal input for the most flexibility, and adds in a kill switch and large decoupling capacitor. This splits out to a direct power output to the motor controller with a motor kill switch to let me test the other parts of the robot without worrying about the motors. I put the cap before the switches so that the capacitor discharge doesn't affect the time to kill the system.
There's a pair of voltage regulators: an LD1117 to provide 3.3V and an LM317 to provide 5V. These have decoupling caps placed after them. They go to a set of headers that provide power connections for the relevant systems.
Finally, there's an isolated I2C expansion that lets me add up to seven I2C peripherals via standard header.
2.0: fixed capacitors (the caps were in-line with the circuit instead of acting as decoupled caps).
1.0: First pass.