Yeah.... Crystal Space, Ogre3D, Panda3D, Blender game engine, Doom3, darkplaces, Tenebrae, etc... There are many good 3D engines / SDKs out there, FLOSS or otherwise.
Yes, Crystal can load Q3 maps.
Still, making a decent game is non-trivial. These engines do not create game content, concepts or even much of the required code. See the Ecksdee project:
http://ecksdee.sourceforge.net/wiki/wiki/Main_Page - there is a video there, one of the main developers gives a talk about the project, including some of the technicalities of using crystal to make a game.