I have exactly such a thing which I would love other people to test for usability. I have advertised it on
http://www.sudokubuddy.co.uk (N.B. I need to sort out this website properly rather than spending all my time solving puzzles). There are seven 'levels' of help, from nothing at all (but convenient boxes for notes and things) to giving you all the options in particular cells and allowing you to focus on the harder techniques yourself. It sometimes solves 'Fiendish' by itself, but rarely. I usually use Level 2 which just checks for error entries (e.g. double numbers in a row/column/box) but nothing else.
Just send me a message at
andrew@sudokubuddy.co.uk if you want it. Don't worry about the fee listed on the website, I'd like to get some more people to test it right now.