http://www.kenken.com/playnow.html
new favorite puzzle game.
I've done a 4x4 in 42 seconds
5x5 in 15 min
6x6 in 48 min
I havn't tried 7x7 yet. word on the street is that really good people can do it without the operation symbol, making that another rule that must be inferred, and leaving them with only the cages and rows/columns as constraints.
I like that the game started from a learning theory, and evolved into a puzzle. A japanese teacher had a theory that we don't really know anything save that which we are forced to infer through our own efforts. He thought preparing the mind for science and math should involve such efforts, rather than textbooks and fact teaching. These Kenken puzzles have only a set of constraints and competing operational areas (the cages with different math operations) forcing the mind to switch between and fit together multiple logical operations while utilizing trial-and-error in an attempt to infer the correct solution. This, the theory goes, trains the mind for math, logic, and science better than the conventional classroom practice of lecturing and teaching.