Some common solving techniques used by human players. (Ref)
Nishio is a guesswork technique. (Kind of slow)
Remodels the puzzle into an Exact Cover problem.
A pair of INVALID 16 clues
The six MOST difficult I could find online
Testing set of the first 39 sudokus from 49,152 sudoku17 file:
Works in all modern browsers.
XHTML 1.1, CSS 2, W3C DOM, ECMAScript 3
Solving features may freeze on old browsers or slow machines.