Sunday's FoxTrot comic strip by Bill Amend tied together at least three things I like: Sudoku (the number puzzle), mathematics, and FoxTrot itself.
The nerdy Jason Fox creates his very own Sudoku with mathematical formulae instead of raw numbers. How much fun to have powers, derivatives, integrals, log, trigonometry, binary and hexadecimal numbers. The next version will require that your answer include no repeated formulae in any 3x3 square or in the rows and columns.
[They clearly don't want people copying the art, so I won't do so. If you are addicted, have a look.]
Update: I placed the puzzle with math solved below.
Print and enjoy.
7 3 9 4
9 4 3 7
6 8
4 3 6
7 1
2 9 5
5 1
6 7 8 3
3 4 1 7