Very simple Chrome plugin for solving the http://www.websudoku.com/ puzzles.
cat hard_board.txt
800000000003600000070090200050007000000045700000100030001000068008500010090000400
cat hard_board.txt | ./sudoku_standalone
812753649943682175675491283154237896369845721287169534521974368438526917796318452
636 9956
812 753 649
943 682 175
675 491 283
154 237 896
369 845 721
287 169 534
521 974 368
438 526 917
796 318 452