I've heard of Sudoku before, but never saw a free online version to play till today. Granted, I hadn't really looked. When I stumbled on this site I figured I'd give it a try. I didn't really know what to expect, but this shit is hard. It took me several hours to get thru one of the "Easy" ones.
Of course, part of the problem might have been that I was only using two of the three rules to play the game. (I missed the part about getting one number in each 3x3 square.) Oh well, live and learn.
http://www.websudoku.com/