Amps that switch on when receiving an audio signal

I see that there are power amps from Cambridge Audio and Parasound that switch out of standby and into operating mode when they receive an audio signal. This seems to me a pretty cool feature -- cool in more ways than one, of course. (Energy saving, etc.) Are there other amps that do this? Are there many? Is this becoming something of a commonplace and I'm just now noticing it?

Thanks.

-- Howard