Continuous rotation servos
There is a special breed of servos labelled as continuous rotation servos. While a normal servo goes to a specific position depending on the input signal, a continuous rotation servo either rotates clockwise or counter-clockwise at a speed proportional to the signal. For example, the Servo1.write(0) function will make the servomotor spin counter-clockwise at full speed. The Servo1.write(90) function will stop the motor and Servo1.write(180) will turn the motor clockwise at full speed.
There are multiple uses for such servos; however, they are really slow. If you are building a microwave and need a motor to turn the food, this is your choice. But be careful, microwaves are dangerous!