I probably forgot to post this here. The Onyx Fire II boards I made are now available for purchase at Amazon!
They provide two switchable power screw terminals for 8V-18V. You can switch them on/off with TTL serial (9.6 kbps up to 2 Mbps,) with RC-style pulses, or with simple high/low logic.
The switching circuitry is rated at > 20A, but the connectors (2.5mm SPOX, and plain 100mil pin headers) are rated for much less (a few amps) so more than about 5A continuous wouldn't be safe.
These are designed mainly for robots that use the Robotis line of Dynamixel servos, and are compatible with the serial bus and command protocol used by servos such as the AX-12 and MX-64T, and they also work in many other situations.http://goo.gl/vaEcL8
Sadly, these are not making me rich, as I don't expect to be selling tons of them, so each one is expensive to make, and Amazon takes almost $10 of each purchase... I do it because I love robots!
You can find the manual here: http://onyxrobots.com/2014/04/29/onyx-fire-ii-manual/