Commonly used in items such as smoke detectors, radio controllers, alarm clocks (to keep the time settings in the case of a power failure/surge), LED flashlights, and short range walkie talkies.  Basically, any device which requires a higher voltage but does not actively draw a large volume of electricity can use a 9V battery effectively.  In the past, most smoke detector manufacturers have recommended against using rechargeable 9V batteries in their products because of the time which the voltage drops on a NiCd or NiMh compared to a standard alkaline.  Recent advancements in the technology of NiMh rechargeable batteries have improved their effectiveness in these roles, and existing rechargeable alkaline batteries always have.