One common (older) type of variable-pitch prop was the two-position prop developed by Frank Caldwell of Hamilton Standard. It used hydraulic pressure to drive it to low pitch for TO, and when the pressure is released, centrifugal counterweights brought the blades back to high pitch for cruise. It's called a single-acting hydraulic/counterweight prop.
Woodward Governor Company later provided a separate governor unit that could modulate the hydraulic pressure to set the blades at any intermediate angle to hold a constant RPM, regardless of throttle or airspeed (climb or dive) changes.
Most modern props use refinements on these basics.
More history - see:
http://www.memagazine.org/supparch/f...e/goldage.html
and scroll down halfway.