RakNet is a networking API that is a wrapper for reliable UDP and higher level functionality on Windows, Linux, and Unix. It allows any application to communicate with other applications on the same computer, over a LAN, or over the internet. Although it could be used for any networked application, it was developed specifically for rapid development of online games and the addition of multiplayer to single player games.
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.