"Chord piano" (downloadable) lets you hear your part against other people's (in vertically stacked boxes) - click in a box, then click the keyboard to set the note for that box. Double left or right arrow buttons move you along horizontally one stack at a time, and what you hear is always what's in the middle stack at the time.
Do you have a music game idea? Share with me your music ideas. If realization is easy then I will made free for you.