Recs.
Updated
Panda3D is a free and open-source game engine developed by Disney and Carnegie Mellon University. It has been used in commercial MMORPG games such as Disney's Pirates of the Caribbean Online and Toontown Online, as well as various indie projects. It is available under the Modified BSD License.
SpecsUpdate
Pros
Pro Python scripting environment
Although it's possible to use only C++ to program in Panda3D, all its power is available to the Python scripting language, while not trading in performance since the performance-critical parts are implemented in C++. It has a powerful binding layer that exposes the vast majority of the API via Pythonic interfaces.
Cons
Con No unified editing program
Unlike Unity and Unreal, Panda3D doesn't currently offer a single, unified editing program in which objects can simply be dragged in and assigned properties (although third-party solutions are available). Developers are expected to design their scenes in a modelling program like Maya or Blender instead, and bring it into Panda3D using Python code.