Moddable

Apocalypse Game Engine (AGE) is a PHP based framework designed to be moddable. It is geared toward turn-based strategy games, and will allow admins to customize settings.It uses MySQL as the database, although it will be ported to allow virtually every type of database. Platforms: PHP

A moddable 2d platformer/side-scroller game engine. Due to the dynamism and abilities of Python all elements of the engine may be subclassed and extended to provide unlimited functionality to the developer. Platforms: Windows, Mac, Linux

Hale is an extremely customizable and moddable tactical RPG written in Java with extensive support for scripting through JavaScript. Platforms: Windows, Mac, Linux

Apocalypse Game Engine (AGE) is a PHP based framework designed to be moddable. It is geared toward turn-based strategy games, and will allow admins to customize settings. It uses MySQL as the database, although it will be ported to allow virtually every type of database. Platforms: Windows, Mac, *nix, PHP, BSD Solaris

OpenRA is a Libre/Free Real Time Strategy game engine supporting early Westwood games like Command & Conquer and Command & Conquer: Red Alert. OpenRA is also a highly moddable RTS game engine, in fact both Red Alert and Command & Conquer are implemented purely as mods on it. The Red Alert mod... Platforms: Windows, Mac, BSD, Linux