http://sudokuhelper.appspot.com
This is the smartest sudoku helper you can find.
Introduction:
This app is to help sudoku players with their puzzles. Next version will provide some build in puzzles to play with.
V1.1 features:
1. Added permanent link for sharing puzzle with friends.
2. Smarter hints.
V1.0 features:
1. Lock and unlock cells.
2. Store and load snapshots.
3. Verify whether a puzzle is solvable.
4. Get a hint.
5. Auto-solve.