Hypersudoku
(aka Windoku or Four Square Sudoku)
Hypersudoku has an intriguing twist. Inside the normal sudoku grid are four more 3x3 boxes that also contain the numbers 1-9 without repeating. In the below example, the boxes shaded green are the overlapping additional boxes that also need to be solved. It is fun solving the puzzle as you switch between the to two sets of boxes.
Example of hypersudoku puzzle grid