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.
GNS, or Game Name Search, is a cross-platform C++ library designed to enhance your application with hosted game searching, product updates, error reporting, chat rooms, and content sharing capabilities. Its features include online searching for hosted GNS-enabled video games, Internet-based chat room support, real-time product updates over the Internet, generic content hosting, program crash reporting (Win32 only), 16-bit Unicode support, and plug-in support for third-party functionality expansions.
jIsoEngine is an isometric 2D engine for Java applications. It provides an SDK of methods and classes that let developers easily build a complete game. It is suited for simulation games, board games, and turn-based games. The project is bundled with Tourist Camping Tycoon, a game developed with the engine.