The Brick Engine is a fast and simple lo-fi gaming engine. The engine provides a comprehensive API, including dazzling visual features, song playback, and collision detection: everything you need to make games with quickness and ease. The engine provides a straightforward C API and has language bindings for Tcl and Pascal, with support for more languages on the way.
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.