A fabulous web resource to both solve sudoku puzzles using logic and to learn advanced techniques for sudoku.  

Scanraid LTD has a web page which will solve that sudoku puzzle that is driving you mad :-)

as well as teach you the advanced techniques of logical analysis so you can solve them on your own!

Give it a try: Sudoku Solver by Andrew Stewart 

To give you a "jump start" here is a one-image "hint sheet" on how to use Andrew Stewart's page: