Donations would, of course, be very welcome, too. If you have PayPal, you can just send whatever amount you want to firstname.lastname@example.org. It doesn't have to be huge, after all, ten bucks is ten bucks.
New feature as of 07 June 2009: can now specify a list of acceptable levels (easy, medium, etc.) on the command line and/or in config.tcl (command line wins if they disagree). Valid levels are "easy", "medium", "hard", and "fiendish".
GT-Sudoku-06Feb2011.zip - This is the full release (06Feb2011 update) of the above Sudoku.tcl (plus). It includes sudoku.tcl for Unix and Mac OS/X users, and sudoku.exe for Windows users (who, if they have Tcl/Tk installed, can, of course, also just use sudoku.tcl).
Unzip the file, edit config.tcl (any plain text editor will do) and run either sudoku.exe or sudoku.tcl (using wish8.4 or, on Windows with the ActiveTcl release, wish84---I have no idea if this code will work with Tcl/Tk 8.5 ... it *SHOULD*, but since I haven't tested it, no promises).