Simplest controller - just a contactor (high current relay) to turn the power on or off. Could have more than one contactor and resistors to enable stepped speed control.
The most commonly used controllers - for DC motors popular manufacturers:
- Dc Power Systems Raptor http://www.dcpowersystems.com/r600fly.htm (out of business?)
Do it yourself schematics: http://www.austinev.org/evalbum/tech/
Curtis-Killer DC controller
My essential spec/feature list for a Curtis"-Killer" DC controller is:
Controllers for AC motors