Y.A.H.G.E. - Yet Another Homemade Game Engine is what the name says a free open source 3D Game Engine geared towards development of 3D First Person Shooters.
Major goals are to provide a Rapid Prototyping System for Game Ideas or to make Game Demos for Resume Samples. :) It is designed with an object oriented and modular architecture . Please check the project documentation and FAQ
fpr more details.