Botcraft is a cross-platform C++ library to create bots
Robocode is a programming tank game for Java
Air traffic control tower and radar simulator (solo + multi-player)
Game engine supporting many 3D/2D formats and graphic effects
Java RTS framework
State Management and Multiplayer Networking for Turn-Based Games
Java Settlers
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Carcassonne clone written in python using pygame
A simple fantasy game
2048 game and AI on JavaScript
A chess like board game