I'm still angry at my coworker, Bill, for this one. We've been on this Sudoku kick for a while now, and able to solve the harder ones with a bit of effort. So he (who has the best times in the office) decided to take things up a notch.
He suggested doing the puzzles one cell at a time, left-to-right, top-to-bottom. With no marking. Grrr, talk about breaking my brain.
The thing is, doing it that way means juggling tons of cells in your head at once, even on the easy puzzles. It's more of a mental workout than I've ever enjoyed getting in the past. (Which probably explains why I'm not so good at chess, now that I think about it...)
Anyway, if regular Sudoku is getting to easy for ya, give it a shot.