This ultrasonic range sensor sends out sound waves at a high frequency and then times how long they take to bounce off of something and come back. This is the same type of system used in backup systems that are integrated into cars. This module has a pretty large range of 2-45CM.
This would be well suited for projects that involve sensing the presence of a person or object.
5 x HY-SFR05 Ultrasonic Range Sensor