Eclipse is a set of C++ classes designed to give beginner developers a head start in developing their first games as well as more experienced developers who don't want to have to write code for mundane tasks like fonts, sprites, and image loading. It provides bitmap fonts, window management, audio capabilities, image loading, error logging, and zip file support.
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.
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.
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.
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.