Ogre 3d is one of the best and free open source rendering engines available. It is written in C++ and is actively being developed. Check it out at Ogre3d.org
Note: Ogre is being updated and restructured in it's newest versions and many of it's initialization procedures and overall usages are changing. The latest 2.0 and 2.1 versions are significantly different from previous versions and require much documentation reading to learn the newest changes.