I had written a Sudoku puzzle solver in Java some time back. Had some time today and rewrote it in Python so I could deploy it on GAE. The result is Sudoku Ninja.
It can currently solve most of the Hard problems on Web Sudoku.
It also has an API where you can enter the puzzle data as a series of numbers row by row and have ‘0’ in place of unfilled cells and send it as a GET request This returns text with the values filled in accordingly. Example.