Emily is enamored of Sudoku puzzles lately, and has gotten me into them as well. I bought a copy of some Sudoku software which has the neat capability of letting you plug in puzzles from books or newspapers to solve. The software will even verify the clues to tell you the difficulty of the puzzle and whether or not the puzzle can be solved using logic (versus those puzzles that cannot be solved without resorting to guessing).
Lately, Emily has been creating her own puzzles, but the Sudoku program usually can't verify them as “logically solvable”. A search for a “creating sudoku puzzles” on Google yielded this priceless tutorial: Let's Make Sudoku! (There's even an advanced tutorial for creating more difficult puzzles).