Sudoku SoftwareI wrote a sudoku generator program, a puzzle editor/solver program and an puzzle analysis tool in QBasic. Compiled versions of these programs can be downloaded here. The latest versions were uploaded on 7th September 2008. (the puzzle generator is much faster). They employ the following solving techniques: hidden and naked singles, pairs, triples and quads, two types of locked candidates, x-wings, swordfish, jellyfish, finned x-wings, finned swordfish, finned jellyfish, xy-chains, xyz-wings, colours on single candidates and on multiple candidates, and nice loops on single candidates and on multiple candidates. You'll find a pdf file explaining most of these techniques here.
Below are screenshots from the editor program and the analysis tool. In each screenshot, the large grid on the left is the candidates grid, the upper grid on the right is the puzzle and the lower grid on the right is the solution.
The sudoku puzzle in the first screenshot has one clue missing to in order to show some candidates in the candidates grid.
The analysis tool steps through the solution to a puzzle. At each step hidden singles are shown in red in the solution and naked singles are shown in green. As you step through the solution you can turn different solving techniques on and off. In the following screenshot from the analysis tool, on the last step one naked single and three hidden singles were entered in the solution grid. With hidden pairs switched on, in the candidates grid you can see two hidden pairs highlighted.