Just added some HC-SR04 ultrasonic distance measuring modules to our stock in the Electronics Area.
This little guy uses ultrasonic sonar to give you a distance reading from one inch to over ten feet. It’s one of the more common sensor modules out there, tons of existing code and examples and tutorials for whatever controller you are using to talk to it. Check out the well supported arduino library at http://playground.arduino.cc/Code/NewPing or just search for “HC-SR04” and you will see tons of data on it.
In the drawer marked “Sonar”, grab one today to play with it and see how far stuff is.