Number Logic
Caught up in the current Sudoku craze? This site offers a hefty supply of puzzles to tackle. Work independently or compete head-to-head against other registered users. There are three levels of difficulty; the site will time you (there's a pause button if you need to take a breather) and, if you wish, "validate" your answers (and highlight any mistakes). You don't have to sign in to use the site — but you do if you want your scores recorded.