Using this app, you can play the piano using either your computer's mouse or keyboard. Holding ctrl on a Mac laptop and playing with your keyboard will shift the sounds down one octave. Signing in or signing up directs you to a page on which you can record the music you're playing and then download it in an .ogg file. Some of this functionality does not translate perfectly to Windows laptops, touch-screens or screens with unusual screen dimensions, but it will soon!
(Warning: Works best on Google Chrome)