Pendula is a single and two-player network game based on intervals. The single player version is guessing game in which the player first hears and "sees" a target interval and tries to guess that interval by playing it back using the computer keyboard. Played musical intervals are visualized with a harmonogram, which is a plot that shows the relation between the two notes. In the two-player version, players compete against each other to find the target interval in a more challenging way: the last two notes they played are used to form the current interval, so a player should choose what note to trigger depending on what his opponent played.
The idea came when I was making my way out in the bookstore and I bumped over a book called Harmonograph, A Visual Guide to the Mathematics of Music