SudokuSolver Forum

A friend of mine who runs a set of forums also has a game arcade as an add-on to the forums. Is there a good sudoku game she can add to the arcade, with a good supply of single-solution puzzles in a decent range of difficulty? It needs to be free or very low cost, as she doesn't charge for forum access.

I personally prefer pencil and paper solving, but I told her I'd give an arcade version a good try.