Hear the beep when you sleep- smoke detectors
New sensor technology as well as battery improvements have evolved a new generation of smoke and carbon monoxide detectors that last for their entire 10 year life without having to install new batteries.
This makes the problem of retro fitting existing houses with dectors extremely simple, especially now that installing a detector in every bedroom has been realized as a sensible thing to do, and will be required by the building code.
One company, Kidde, manufactures units for various areas-kitchen, high ceiling, combination carbon monoxide and smoke detection, as well as units with lights built in.
Other than vacuuming them periodically there is no maintenance required for their entire lifespan, and most importantly , they function during a power outage when people are more likely to have a fire start from the use of candles.