D'Enfent Engine is a library that contains high-level classes to access OpenGL functions. It contains classes that may be very useful in game development: GUI components, sprite classes, and classes to handle mouse/keyboard events. It has a full API reference, as well as lessons to help you start working with the library.
Sudoku Sensei is not a Sudoku solver, but an "explainer". It can list the logical steps taken. It can generate new Sudokus and rank their difficulty. It supports not only 9x9 grids, but MxN, samurai, or any layout you can describe with the board editor. It has a GUI, manual, tutorial, and examples.