Create any number of Sudokus and solve them. Store Sudokus for in-depth study.
Solvability guaranteed. Drag digits from the palette or click on the board
and wait for the desired digit to appear. Neither method will let you place
double digits.

Includes automatic markup.
You can input external sudokus for solution.

WWW: http://www.gnustep.it/marko/Sudoku/index.html