0 A.D. (pronounced "zero ey-dee") is a cross-platform real-time strategy (RTS) game of ancient warfare. It's a historically-based war/economy game that allows players to relive or rewrite the history of ancient civilizations, each depicted at their peak of economic growth and military prowess.
Andrew's Game aims to be a lightweight and highly expandable code base editable by anyone with a basic understanding of Python. The project has three parts: core systems, sample game, and reusable extensions. Core systems deal with the actual game mechanics; a sample game will be a template of how to use core systems and reusable extensions to create a working game. The reusable extensions can contain items and minor added functionality. Andrew's Game can implement a text-only single player RPG, which is like a MUD without support for multiple players.
Arx Libertatis is a cross-platform, open source port of Arx Fatalis, a 2002 first-person role-playing game developed by Arkane Studios. Arx Fatalis features crafting, melee and ranged combat, and a unique casting system where the player draws runes in real time to effect the desired spell. The Arx Libertatis source code is based on the publicly released Arx Fatalis sources. This does however not include the game data, so you need to obtain a copy of the original Arx Fatalis or its demo.