go for broke the Unreal 4 engine.

I whole-heartedly suggest OpenSceneGraph. It's been around for ages, it's used by NASA and Boeing, it's released under a slightly-modified LGPL, and supports OpenGL 1.0-4.2.

It's actually not a "game engine" itself, but it provides rendering, input, animation, GUI, et cetera, and forms the basis for the Delta3d engine. Combine it with your favorite physics engine (there's a community project integrating it with Bullet), add scripting (community bindings for Java, Lua and Python, though I suggest checking out AngelScript), and you've got most of what you need.

I'm using OpenSceneGraph for my own game, and I love it. It's enough to get things going, but leaves room for you to do some of your own programming. I'm actually working on creating my own game framework with an editor like Unity has (but way better, because mine's developed on Linux). If there's any interest in it, I'll probably release it under the LGPL or something.

I may get shot down for this but... What are your thoughts on Unreal?

That is a seriously good question in my opinion, one of which I would like an answer to as well. My programming experience doesn't fall within games, but I am very much interested in knowing if this does or does not work as a suggestion, and why. Skwee seems to know his stuff, and what he's looking for, as well as why. Getting some feed back from him, if not someone who has worked with Unreal Engine would be nice.

Per wiki comments:

The current release is Unreal Engine 3, designed for Microsoft's DirectX 9 (for Windows and Xbox 360), DirectX 10 (for Windows Vista) and DirectX 11 (for Windows 7 and later); OpenGL (for OS X, Linux, PlayStation 3, Wii U, and iOS), Android; Stage 3D (for Adobe Flash Player 11 and later); andJavaScript/WebGL (for HTML5).

Source: http://en.wikipedia.org/wiki/Unreal_Engine

License

Proprietary; UDK free for noncommercial us

Written in

C++ , UnrealScript

EDIT: Whoops, looks like I didn't see their were 2 additional pages full of comments after Godmil's post. Sorry about that.

grit is very good, it's written in C++ and uses lua for it's scripting language it is designed for developing open world games like GTA you can fined it on www.gritengine.com

Grit is based on Ogre :-)

There is also Crystal Space which, together with its sister projects, might be worth checking out.

Irrlicht 3d Engine is full open source good and free
Proton 3d Engine kernel use Irrlicht also full open source
Build on Win32 Macox Linux Android Ios
try this https://github.com/fatalfeel/proton_sdk_source
and svn rtsoft.com/rtsvn/proton
and svn svn.code.sf.net/p/irrlicht/code/branches/ogl-es?

Please don't necro threads.