A sudoku solver, puzzle generator and GUI.
I wrote this for a Linux Format bounty in 2005 (which it won). I haven't touched it much since then. I'm not 100% sure but this might be the first instance of the row/column/cell highlighting UI feature for interactive sudoku solvers that has been copied a fair bit since then; if anyone knows of an earlier example let me know (or possibly it's an obvious idea that multiple people have stumbled across).