Ron Jeffries, of Extreme Programming fame, and some other folks on the Test Driven Development email list, have started several passes at creating a Sudoku solver by following the TDD approach. I really am enjoying following Ron's pursuit in his article, OK, Sudoku, and in the forum and list messages. Ron is doing a Ruby version, which is even more fun to follow. If you want to see TDD in action, watch as Ron goes about implementing something about which he knew basically nothing when he started. Great fun!