Few years ago, everybody believed that it would take thousands of lines of code and years of work needed to create a viable game. The time has changed now and with so many game engines available, development of game is as easy as it sounds and i am a great fan of open source game engines.
Please keep in mind that a game engine usually consists of graphics engine, sound library and server for multiplayer. The game engines listed below are graphics engines capable of rendering realtime 3d environment.
1. Irrlicht
http://irrlicht.sourceforge.net
Irrlicht is a a lightning fast free open source high performance realtime 3D graphics engine written and usable in C++ and also available for .NET languages. It is completely cross-platform, using D3D, OpenGL and its own software renderer, and has all the best features which can be found in commercial 3d engines. The engine is really fast and has excellent c++ documentation and APIs. There are also numerous tools available for game development with irrlicht.
2. Ogre3d
Ogre3D is an open source graphics engine and has recently emerged as the popular game engine used in simulators, games and educational projects. It supports both directx and openGL and in windows, linux and macosx platforms. The game engine has extensive API documentation.
3. Genesis 3D
Genesis 3D is a open source game engine for for realtime 3d game rendering environment. It has a flexible license designed to allow use of the game in commercial, or non-commercial use. The game has large user community and number of tutorials available for developers.
4. UNITY 3D
Unity 3D is a easy to use game development framework for absolute beginners. Unity has highly optimized graphics performance and it supports various platforms PC, Mac including iPhone, Wii, iPad. The license is a free but pro version is only available as a paid option.
5. Unreal Development Kit
Unreal development Kit (UDK), powered by Unreal 3.0 engine is now available for game developers under a very flexible license for both commercial and use of non-commercial purpose is free of charge. The engine is free for educational purpose as well. It is complete development framework to create realistic gaming world with ease of use.
6. Panda 3D
Panda3D is a free 3D rendering and game development framework for Python/ C++. Panda3D is Open Source and free for any purpose, including commercial projects. It is a mature and complete development kit for game developers with a number of added features for graphics rendering.
7. SIO2 Engine
http://www.sio2interactive.com
Free open source 3d engine for iphone, ipad and ipod touch for game developers.
8. 3Impact
Free 3D SDK for development of PC games. It requires directx 9.0 and above and games can be developed with the shortest learning curve for rapid game development. It is free to use and no royalties involved. The engine supports both 3d and 2d rendering and has number of features.
9. Cipher Game Engine
Cipher is a start of the art game engine to create powerful and good performance 3d games for game developers. The engine is a commercial and can be purchased for small fee.
10. C4 Engine
http://www.terathon.com/c4engine/
C4 Engine is advanced 3d game engine available for Windows and Mac OSX platforms. It comes with integrated support to audio, input, physics and much more.. The engine is not free but for a small fee you can get a basic version of this engine.
11. Truevision3D
Truevision 3d is game development SDK and middleware packed with 3d engine, sound engine and video and networking engine all in one package. The engine is only available as commercial product but the good news is, it can be purchased with small fee.
There are number of game engines available besides these and if you are looking for more engines, please visit this wiki page
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