Sudoku Susser
Here’s a sudoku app that goes out of its way to help you solve the puzzle with tons of hints and shortcuts. It comes with several puzzles, but you’ll want to find more and import them. One method — amazingly enough — is to take a screenshot from a website such as this one and drag it onto the app. It actually analyzes the numbers and creates a puzzle based on it. Very impressive. Sudoku Susser is freeware, but the developer makes a strong plea for donations.