Like previously said Ogre3d and Irrlicht are probably your best options in regards to "free". If you also want "easy to learn" it boils down to Irrlicht.
Very clean interface and great tutorials.
Edit: Actually you should be a little bit more specific. Do you want 2d or 3d graphics? Do you need sound, physics and all that or just graphics? Which platforms do you want to target?
Also, are you sure it has to be C++? While C++ is pretty much mandatory for professional game engines, for learning purposes C# with XNA or some scripting language is way better.