Piezoelectrics are used both commercially and industrially. Commercially, their most common use is as gas lighters. These are capable of producing a spark, as in this animation:
Industrially, they are mainly used for imaging, mostly in medicine. They are used to produce ultrasound, which is used to check on unborn babies. In a non-medicinal manner, it can be used to detect cracks.
However, this effect can be utilised, without generating the wave with the piezoelectric. In this way, the piezoelectric is used solely as a mechanical sensor. As it picks up a mechanical deformation, it generates a voltage, and this can be detected, allowing them to be used as sensors.
Another very common use of piezoelectrics is in watches. A small piece of quartz crystal is used to regulate the movement of hands, as its shape will oscillate at some known frequency when a particular voltage is applied. This oscillation can be translated into a very accurate timekeeping device. For more information, see this external web page: http://electronics.howstuffworks.com/quartz-watch.htm
A final possible use is that of an actuator. If the electric field applied over a piezoelectric is not oscillated, but instead simply applied, the change in shape of the piezoelectric can be used to move objects. This is useful in micro-scale positioning, as the change in shape of the piezoelectric can be measured in microns.
For more information, see: www.morganelectroceramics.com/piezo_products.html