Variants

Eijidoku
Word based version of the game.
http://www.eijidoku.com/

Killer Samurai Sudoku
Offers printable samurai sudoku puzzles for download. Also provides rules and solution strategies.
http://www.killer-samurai.com/

Math Games: Sudoku Variations
Offers detailed descriptions and illustrations of Sudoku variations, including Diagonal, Cubical, Even-Odd, and hybrids. Also includes a short history.
http://www.maa.org/editorial/mathgames/mathgames_09_05_05.html

Mathematical Recreations
Custom Word Doku puzzles devised by Steve Schaefer. Features an entire sentence embedded into the puzzle.
http://www.mathrec.org/sudoku/

Sudoku Samurai
Offers printable sudoku samurai puzzles for download.
http://www.binaryworlds.com/sudoku/

Sudoku-Ball
A three dimensional version of the puzzle. Includes game rules, online version and news.
http://www.sudoku-ball.com/

Sudokuto
Two-player variant. Offers several board sizes to play against the computer. [English/Norsk]
http://www.menneske.no/sudokuto/eng/