SmartMusic's tuner has a clever feature: it plays back the note it thinks you are trying to play.
If you want a laugh, turn it on and try speaking to it. It will try to detect pitches in your voice. (a distant cousin to auto-tuning I guess).
How about a tuner playback that remains silent as long as you are within certain parameters?
You can set how "narrow the goalposts" should be...5 cents, 15 cents, 20 cents, etc.
One of my students pointed out that this could be turned into a game sort of like that old classic, Operation. As long as you're in tune, the patient doesn't suffer!
Yes, I posted something similar in 2005.