Wesnoth

The Battle for Wesnoth is a turn-based tactical strategy game with a high fantasy theme. It features single player and online multiplayer combat. Its GUI and gameplay graphics are impressive as well as its multi-platform support (it even builds on NaCL). It has clean, well-coded examples of pretty much everything a game developer would want to know, from a an embedded lua scripting engine, to a dialog and GUI system, to a c++ test suite and cross-platform builds.