The MaxSonar-EZ1 Ultrasonic Range Finder is a tiny, easy-to-use ultrasonic ranging system that can be easily interface to any microcontroller.
The MaxSonar-EZ1 is one of the smallest, most efficient, sonar (ultrasonic) ranging systems available. The MaxSonar-EZ1 can detect objects from 0-inches to 254-inches (6.45 meters / 21.1 ft.) and provides sonar range information from 6-inches out to 254-inches with 1-inch resolution. The interface output formats included are pulse width output, analog voltage output, and serial output (TTL).
For further comparisons between the various sonar sensors MaxBotix has, check out their comparison chart.