Whatever things are true, whatever things are noble, whatever things are just, whatever things are pure, whatever things are lovely, whatever things are of good report, if there is any virtue and if there is anything praiseworthy -- meditate on these things.
This is the hardest one I have ever worked on. [Update 10/24/2006: thanks to you all reading this post and posting other links, I have since found more interesting ones -- look through the comments - the one posted on October 9th appears to be pretty hard, though maybe I just made a mistake on it] When we were with my parents last weekend, we found out that my parents are big Suduko fans, and do quite a number of them during the week, in different papers, page-a-day calendars, etc.
Dad was trying to find a hard one for me, and found a "six star" one, that turned out to be not that hard - reasonably difficult, but similar to what I had seen before. We had been trading secrets of how we figure out puzzles the fastest, and he liked one of my starting first-pass rules, but then found a puzzle that using his rule, and then my rule still did not find a single number during the first pass. After that, the only step I know is to start writing down all possibilities, and it is sort of interesting to see how fast you can narrow them down, it is a little too brute force for my taste, and so I am not interested in it as much.
Once you get one number on this puzzle, it is a normal, reasonably difficult sudoku, the trick is getting the first number. So, I would be interested in hearing how long it takes you to get the first number filled in, and if it doesn't take you that long, what is your strategy, because it must be different than mine.(see my comment on 11/27/2006 for the original puzzle)
Edit: Don't read the comments if you are interested in solving the puzzle, wait until after you are done, or at least until after you have found the first square -- which Linda and I probably had the same first square, so that leads me to believe that everyone might have to start at the same point.