My Sudoku Secretary is offered in the hope that it may be useful. It is quick to type in your puzzle, and at its simplest you can use SS simply as a way of printing out a nice clean copy at intervals during your solution. Or, to save a bit of donkey work, before you print the puzzle, SS will add those small numbers to each blank square, showing those which are not in its row, column, or box. Finally, and this is if you're really stuck, SS will fill in 'unique values' - that is to say it checks each row, each column, and each box to see if only one number will fit; you can do this all at once (which immediately solves moderate and below), or one at a time, until you get on track again. Here is a preview of the help that comes with the product.
Download instructions: just follow the link above and keep saying Yes, or OK. When it's done, there is a new picture to click on your desktop. I promise I won't infect you with a virus, even though you may see a box saying I am uncertified. If you need a character reference, you could look further round this website.
You can just download Sudoku Secretary, and not bother with the following reflections, but here they are:
The thing about Sudoku is that it is trivially easy to solve by computer - a google will show millions of programs that will do the job. This is because a computer is good at copying things, and patiently looking through things, without ever making a mistake.
My Sudoku Secretary, though, is not such a program. Its aim is to keep meticulous records for you, and only offer such assistance as you may ask for. It will never do anything clever!
I thought at one time that a Sudoku puzzle was ultimately unsatisfying because its solution will always yield to painstaking mundane analysis: we note, for each square, the possible occupiers, then look for unique answers and ambiguities; for each ambiguity, copy what you've got, take a view, see if it works, and, if not, go back & try another leg of the ambiguity.
Then I decided this was true, but uninteresting. A horse, after all, can run faster than a man, but it is still interesting for men to compete one against the other: and just so can we approach Sudoku - not worrying about the quick, boring computer solution, but exercising human ingenuity. What my Sudoku Secretary is for, is to be an unobtrusive and reliable assistant. It does the hack work of writing down possibilities in the squares; it preserves your work; and it leaves you free to pursue the grand strategy, with a printout of the latest state when you're away from the computer.
Give it a try - it's pretty intuitive and fully described in the help.