Hitori is a Japanese logic puzzle game, similar to Sudoku, but with a different set of rules. The objective is to fill a square grid with numbers so that each row and column contains only one instance of each number. The rules of the game are that each row and column must contain exactly one of each number from 1 to the number of squares in the grid, and no two adjacent cells (horizontally or vertically) can contain the same number. The challenge is to figure out which numbers to fill in while following the rules. The game can be played on paper or on an app or website.
Discontinued It seems that the project is no longer updated, but the latest version, 2.2.0, released in July 2012, is fully working and can still be downloaded from the official website.