Delta3D is a widely used and well-supported open source game and simulation engine. Delta3D is a fully-featured game engine appropriate for a wide variety of uses including training, education, visualization, and entertainment. Delta3D is unique because it offers features specifically suited to the Modeling and Simulation and DoD communities such as High Level Architecture (HLA), After Action Review (AAR), large scale terrain support, and SCORM Learning Management System (LMS) integration.
Delta3D is an Open Source engine which can be used for games, simulations, or other graphical applications. Its modular design integrates other well-known Open Source projects such as Open Scene Graph, Open Dynamics Engine, Character Animation Library, and OpenAL. Rather than bury the underlying modules, Delta3D integrates them together in an easy-to-use API — always allowing access to the important underlying components. This provides a high-level API while still allowing the end user the optional, low-level functionality.
Download: Delta3D 2.1.0