The 7805 ( and the 78M05, the 78L05, etc.) is a 5V voltage regulator integrated circuit. It provides power to other chips on the board that need 5V to operate.
Many of the electronics boards used to control RepRaps and RepStraps -- including the Stepper Motor Driver 2.3, the BuildingAStripboardCommsController, Universal Controller Board 1.2, etc. -- have a 7805 (often the 78L05 version) soldered to it.