If you are flying a single-engined light aircraft, you don't get a lot of choice where to land if the engine stops at low altitude. Engine failure during take-off or landing, or above built-up or otherwise difficult landing areas, is particularly troublesome.
The traditional solution to this is to have another engine (or more). But adding engines adds a good deal of cost and complexity to the aircraft, and probably hurts fuel efficiency too. The asymmetric thrust distribution can be difficult to cope with if an engine does fail on one side, perhaps leading to this nugget (thank you Ross): "What's the second engine for on a plane? To get to the crash site more quickly."
OK I don't claim that this is necessarily a sensible or safe alternative, but it's fun. Imagine a single-engined plane, but with a centrally-mounted solid rocket thruster, perhaps bolted underneath the fuselage or pointing out of the back. This would lie dormant for the lifetime of the plane, hopefully. But in the event of engine failure, a big red button (or even automatic circuit) would ignite it. This could provide a modest part-throttle thrust performance, with no control probably, but good enough for perhaps a few minutes of controlled flight to find a decent spot to land. Some way of jettisoning or extinguishing it after landing might be handy too...