Macdoku is a free and open source sudoku puzzle generator, solver, and assistant. With Macdoku, you can easily manage the possibilities available to each square. The three main possibility constraints are updated live as you type, and other possibilities can be managed in elimination mode. Highlighting of squares with one remaining possibility can be toggled on and off. Pressing TAB will now automatically fill in all such solved squares.  It uses qqwing to generate  new puzzles with various levels of difficulty, and sudoku-solver-in-c as its solving engine.
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Version 0.6.2 (Released Dec 2010)