The app is called KeyScreen and this is what makes it different from the rest:
1. Full customization on the keys shown (you control exactly what keys are shown)
2. Default and custom themes for the keys
3. Support for different keyboard layouts, not just QWERTY
4. Handles multiple displays seemlessly
5. Follows Apple's very own design style
I am now planning to add a feature where users can choose the apps for which the keystrokes will be shown. This is again something that no other apps have. Having said that, I feel this is the best time to suggest additional features if your needs are different. I will be happy to add them.