Mini Piano Pro is a great multitouch piano for Android.
It's designed to be very fast and responsive, and it allows you to choose a beautiful sampled piano or select one of the 128 different midi instruments.
You can use 5 octaves to play and zoom in and out to get the keys to the size you want them.
You can record your songs as midi files and use them as ringtone.
If your device supports it, you can use all your 5 or 10 fingers at once.
This Pro version has no ads, so more space for your piano keyboard.
Volume can be changed with your device's volume buttons.
App is also optimized for tablets.