Airliners are often (almost always actually) pushed back by a tug or a tractor to leave the gate. The tug can also tow an aircraft that cannot taxi by itself.
How much force is required to pull or push a large airliner on the ground? In other words, what is the horsepower of these tugs?
EDIT: By large airliner, I mean Boeing 747 / 777 / A380 type, the ones which ATC append "Heavy" (or "Super") to their call sign.