This solver (and the whole site) is the best one I know of, and it will solve a puzzle by using rules in order of difficulty, to guarantee it tries the simple rules you know before resorting to backtracking.
The benefits of this are that you can grade the difficulty of a sudoku board. You can also solve a puzzle up to the "crux" and then work on only the hard moves manually. I like doing that so I can skip the hours of boring stuff and practice doing the more tricky inferences. Andrew's site is a window into how big of a rabbit hole Sudoku can be...
Check out http://sudokuwiki.com/sudoku.htm
This solver (and the whole site) is the best one I know of, and it will solve a puzzle by using rules in order of difficulty, to guarantee it tries the simple rules you know before resorting to backtracking.
The benefits of this are that you can grade the difficulty of a sudoku board. You can also solve a puzzle up to the "crux" and then work on only the hard moves manually. I like doing that so I can skip the hours of boring stuff and practice doing the more tricky inferences. Andrew's site is a window into how big of a rabbit hole Sudoku can be...