If you're trying to throw together a Botmobile or similar RC setup on the cheap, you can make your own brushed DC speed controller out of the guts of an old servo. I modified the guts out of a Hobby King TGY-S4505B servo for Thingiverse user Luis to use with his Botmobile and turtle shell racer (seen here).

All you need to do is remove the controller of a standard servo and replace the potentiometer with two equal value resistors (typically 2.2k, but higher values should be OK) and hook up the brushed motor you want to control in place of the servo's DC motor. The DC motor you hook up should turn forward or backward at a speed proportional to the throttle on your particular RC setup.

If you've modified a servo for continuous rotation this should be very familiar to you because that's essentially what you've done, you're just not using the mechanics of the servo. You can find some more pics and a write up on my blog here:

http://www.built-to-spec.com/blog/?p=768