Very simply, I go bored shortly after Christmas in 2005 and decided to knock together a program to solve Sudoku puzzles. It isn't very hard, and it took me about an hour. Here's the program in case anyone wants to look at it; it's written in Python. All you have to do is run it and then type in 9 lines of numbers with spaces for the blanks (remember to hit Ctrl-D or similar when you've typed in the last line). Alternatively create a file and redirect it.
If you come across a Sudoku puzzle that my program gives up on, get in touch; I'd be interested (the program certainly doesn't do anything clever, I just wrote it to solve a couple of puzzles in a book I had).