Ultrasound ranging on Arduino
I've recently bought a sonar circuit for distance and temperature measurements. Actually you need to know the temperature if you want to get an accurate distance measurement as it affects sound's propagation speed.
Less than $20 shipped, the URM37 unit is compact and full of interesting features: It has a dual interface as it can use RS-232 levels or TTL levels. Be careful because it defaults to RS-232 and these voltage levels can damage many 5V devices, including Arduino inputs.
It has several operating modes but I've used the serial interface. URM37 library comes with a modified software serial port library that allows you not to use any two available digital pins to interface with the sensor. This way arduino hardware serial port is free to be used for code uploading.
Another cool feature of the sensor is that it can control a RC servo so you can mount the sensor in a mobile platform to get the distance measurement at different angles.