A free app for Android, by Derek Kam.

Music Trainer is a simple and practical app that can help you learn to play the piano. It is based on a concept called flash cards. In this app, you can choose from seven different clefs:

Piano: This is the most common one. The keyboard of the device is divided into seven keys. Each of them has a corresponding note. When you choose the piano clef, the app will show you a random note from the keyboard. You have to identify the note and click on the corresponding button. If you’re correct, the next note will appear. If you’re wrong, you have to try again. It’s a very simple but practical tool.

The app has several different modes. You can choose between random mode and mode with the seven available clefs.