If you've never heard of them, Kakuro's are a type of mathematical puzzle game where you have to enter numbers in a cell to complete a grid.
Kakuro Nichiyou is a freeware variation on this genre and if you've never played it before then prepare to be puzzled as it takes some understanding. The board you are presented with on startup is covered in blocks with two different numbers on them but some of them are blank. The object of the game is to fill all of the blank cells with the numbers 1 to 9 using the blocks surrounding them as clues.
It works in a similar way to a crossword puzzle as you're supposed to use the surrounding blocks as clues as to what should go in the empty boxes. It's quite a challenge for someone as poor at maths as me and in the end, I found myself resorting to the solver to help me get through it. You can also instruct the game to warn you if you've made such a mess of the board that there is no solution remaining. There are also 1 of 7 levels to choose from although I didn't get past 1.
If you're sick of Sudoku and want to try something a little more challenging, Kakuro Nichiyou could well be for you.