Perfect Piano is a free Android app that turns your device into a little piano you can play wherever you go.
Play online, with friends, or by yourself
Perfect Piano has 6 modes: learn to play, keyboard, explore songs, multiplayer, contest, and records manager.
Learn to play helps you to learn piano through simple songs like Greensleeves. The keyboard allows you to play freely and record yourself. Explore Songs lets you see what songs are available to play along with. Multiplayer lets you create or join a room online, where people can play together. Contest lets you play along with songs, Guitar Hero-style, where the more accurate you are, the more points you get. Records manager lets you see and listen to what you've recorded in Keyboard mode.
Basic, but fun and free
Obviously, Perfect Piano is no replacement for the real thing. Tapping away on a touchscreen is nothing like an actual piano. Despite that, it's simple, engaging and can teach you at least the basics of piano playing, if little about the subtleties.
Perfect Piano is also useful if you like to record melody ideas you have on the move. As long as you have reasonable expectations of it, Perfect Piano is pretty good for a free app. If you're looking for an even more basic piano app, try
A keyboard in your pocket
It may not sound amazing, and despite its various modes, it is pretty basic. But Perfect Piano is a neat little keyboard app, and if you like just messing around on the piano, there's no reason you won't enjoy this too.