I know of a few...
The Merlin helicopter has the facility to shutdown one of its engines in flight to save fuel (you are not running the gas generator of the shutdown engine and the other two run at higher power and so higher efficiency). It would not be sufficient to just put the engine at idle to save fuel incidentally. The aircraft is maintained above 90kts to help with restart (the compressor freewheels in the airflow).
I have heard of an A109E having both its engines turned off in flight and successfully relighting. I know of an exercise on test pilots course (see excellent old BBC series on YouTube) where single engined helicopter intentionally does full shut down and restart to practice assessing this. I think 3700 ft was used as a minimum.
So if in a single engined helicopter make sure you can land on what is under you!