Michael J. Mahon & Scott Hemphill have collaborated to produce a Sudoku Solver for the Apple II.

Sudoku Solver runs on any Apple II with 80-column firmware and a 65C02
processor, which includes the IIc, the IIc+, the Enhanced //e, and
the IIgs. A 6502 version is in progress, so that it can run on ALL Apple II machines.

Update: Sudoku Solver has been updated to 2.0 and is now 18% faster. It also now works with any 64K equipped Apple II.