Sudokus
Sudoku-X
Cage-Sum (aka Killer) Sudokus
MiniSum
Downloads
Christmas Puzzles
Sudoku-X - Easy (top), Medium (middle), Hard (bottom)
See also the small variant based on sums - click here or on MiniSum above.
In standard Sudokus, one aims to place digits 1 to 9 in each small square so rows, columns and 3x3 mini-arrays (surrounded by bold lines) contain each digit once.
With SUDOKU-X puzzles (as here), it is also required that each of the main diagonals contains each digit once. A 'correct' Sudoku-X will only soluble if use is made of the additional diagonal requirements (i.e. you can't fill it in fully if you treat it like a standard Sudoku).
The Books page lists collections of both types of puzzles. The Downloads page offers a free Windows-compatible Player program for Sudoku/-X puzzles, as well as downloads of the puzzles featured here.
I also set Killer Sudoku puzzles (which, despite the name, are not necessarily difficult!). These involve Sudoku grids in which no squares are pre-filled in but in which meshes are used to surround given areas, the sum of all contained square values being indicated.