I'm a big fan of the comic strip FoxTrot and today's strip is classic.
In it, Jason (the geeky youngest child) creates his own Sudoku game where instead of having numeric clues to start, the clues are various different math problems. There are square roots, cube roots, numbers in base 2 and hexadecimal, summations, integrals, and more. His sister aptly calls it "Sudorku".
Well, being a huge Sudoku fan and a bit of a geek myself, I had to try to solve it. First, I got to do all the math and dust off a little of gray matter that hadn't seen the light of day in a while (product management doesn't require integrals very often :)). Then it was a pretty straight forward Sudoku solution of medium to hard difficulty.
I've also recently started doing Killer Sudoku which adds a new twist to things. There are actually a few different variations, one that starts with no starting numbers as clues, but instead adjacent cells are grouped and a sum of those cells is indicated. In another there are no numbers at all and instead cells within a 3x3 box have a > or < between them indicating whether the values is greater or less than each other. And then they have a combination of the two.
Try them and have fun!