There is a mini-industry springing up to sell sudoku in a variety of new forms. A number of software makers are introducing versions for cellphones and personal digital assistants, including popular models made for Palm Inc. Retailers such as Discovery Channel Store are introducing hand-held electronic sudoku games, and board-game makers like Millburn, N.J., based Briarpatch Inc. are coming up with their own versions.
Web sites such as sudoku.com are offering premium services where players order an unlimited number of puzzles to play online for around $15. Other sites are also launching new twists, such as a two-player version from number-logic.com that allows players to compete head to head.
Sudoku is even showing up in math classes, thanks to a growing number of Web sites that make it easy for teachers to download age-appropriate versions of the puzzle.