http://home.arcor.de/r.sudogu/sudogu.html
I'm not related to the author of SudoGu, but have the impression that SudoGu is little known outside the German speaking community and therefore I take the occasion to post about it. With version 3.9 poublished a few days ago, SudoGu can finally operate with an English UI beside the default German UI. The web site is still German only.
Just to position SudoGu:
SudoGu is a free Windows only generator/helper/solver written in C# that supports a huge number of Sudoku variants and includes almost all imaginably solving techniques. Yet it has a very simple user interface compared to other "high end" solvers. So it is a program that may appeal to both beginners and experts. The author, Rudy Lars, is very reactive to feedback or bug reports and updates to the program are posted every couple of weeks.