A thousand thanks to Kristin who has posted the link in my comments box. Check out this absolutely, mind boggling Fiendish Sudoku website! They provide 5 puzzles of different levels of difficulty each day. I can assure you that it will take up a fair bit of your time to solve all 5! I would say the puzzles from the previous link which I have given you probably fall under the catergory of easy. Hard takes slightly more time and Fiendish is, well, the name speaks for itself.