Sudoku puzzle solver in C. Based on the strategy described at http://norvig.com/sudoku.html but with more optimized data structures.
On my MacBook Pro (Intel Core 2 Duo 2.66 GHz, running Mac OS X 10.6.8) it solves the puzzles in big.txt (11236 among the hardest sudokus) in less than 6 s compared to 273 s for the original Python version (Python 2.7).