Sussudio is a bare bones sudoku solver. It was built as part of the FreeCodeCamp challenges.
Sussudio accepts incomplete sudoku puzzles with . characters in place of the unknown digits. You can either test the validity of a character at a coordinate, or ask Sussudio to solve the puzzle completely.
A working demonstration is hosted at repl.it, and you can visit it and try it out using the visit website button at the top of this page. The code is hosted at Github and you can visit this using the github button, also at the top of this page.