As mentioned there are two ways used for pushback: towbar and towbarless (TBL) push trucks.
There is a third way that is not as common: the Power Push Unit (PPU).
Source: airliners.net
Unlike the standard pushback process that uses a pushback truck and a towbar attached to the nose wheel, the PPU is a remote controlled truck directly attached to one of the main landing gear bogies. It pushes the aircraft backwards and the pilot steers the nose wheel based on the instructions of the PPU operator.
