Our newest product info video features yours truly explaining the basics of ultrasonic range-finding sensors. If you ever want to add distance sensing, or object/motion detection to your project, these sonar sensors are a great option. They're ultra-easy to use, with an analog output (among a couple other output options), you can get one distance-finding in no time. On top of their ease-of-use, I really like the flexibility of these sensors. You can implement them in both 3.3V and 5V systems (they operate from 2.5-5.5V), and the three different output options mean you can almost always find a pin for them on your microcontroller.
We get all of our range finders from MaxBotix, and they're available in two series – XL and LV. The XL sensors are fairly new and are described as 'super high performance'. In comparison to their LV counterpart, the XL rangefinders emit a higher-power sonar wave (which affords them an extra 4 feet of sensing range), are less susceptible to noise, and they feature real-time, automatic calibration.
Both series feature six different sensors: EZ0, EZ1, EZ2, EZ3, EZ4 and the WR1. Each EZ# sensor has a progressively narrower beam; the lower the identifying number the wider the beam. The more sensitive EZ0 is well-suited for object detection in a wide-open area, while the EZ4 is better-off being used for very directional sensing, or in more cluttered spaces. And then there's the wicked, rocket engine-looking WR1, which has a very precise, narrow beam. The WR1 is ruggedized and designed with outdoor environment usage in mind.
Ultrasonic distance sensors have seemingly endless uses. My college senior design team created an electric water fountain controlled via eight ultrasonic sensors. Others have used them as a part of a flame-throwing trampoline, or a trick-or-treater detecting jack-o-lantern.