Sudoku Online : Sudoku Helper

Sudoku Online : Sudoku Helper: "To really show what goes on when the script checks your board the Take Step button runs through four tests:
Rows and Columns - fills in the blanks with possible numbers by looking at the existing numbers in each row and column. If only one value is possible its written to the board in a large blue colour.
Each Box - does the same as the first test but it looks to eliminate possibles by checking all the existing numbers in each of the nine 3 by 3 boxes.
Pairs Test. We know we can eliminate numbers if two squares have two identical pairs of numbers. For example, if a row has 3-7 and 3-7 then all other 3s and 7s in that row can be eliminated. Same goes for the box and column.
Box/Line reduction: This is part of the Box tests but I've seperated it so it can be seen. We check the box against the rows and columns that intersect it for each number. See here for more.

The actual tests are slightly more in depth that I have space to explain here. Doubtless there are other strategies that this helper can employ and these will be added in the future. The purpose however is to help you check that your own paper and pencil strategy hasn't introduced any mistakes. "

Sudoku is just too much fun. The real test is to evaluate all the rows, columns, and boxes to eliminate as many candidates as possible.