Kanon
Kanon is a somewhat famous Japanese visual novel published by Key. The original version was an adult-only game for the PC. Later, versions for all ages were released for the PC as well as the Dreamcast and PlayStation 2. One of the main reasons for its fame is its intricate plot.
A 13-episode anime series has also been aired, and an OAV, Kazahana has been released.
Its name, like other games released by Key, is derived from a musical term. Kanon is the Japanese katakana spelling for canon.