The following site has extremely difficult grids (XDGs)
http://www.sudokusolver.co.uk/
They call them "very hard" but they require x-Wing & 1st & 2nd order swordfish
(2nd order for me means a 2nd complete check for patterns after a complete 1st pass w/ all chains cleared)
Unfortunately there are only 18 unique grids on that site.
So i came up w/ an algorithm for XDGs (all logic bound)
I now have 800+ million XDGs. Not free.
I also have an MS Access Sudoku Helper
It does not solve the grid, it only assists
in the identification of patterns by highlighting. Not free.
XDGs & associated algorithms are not for the faint hearted
so don't try their 18 freebies @ home, i'm a professional (OK, now LOL)
