Some objects created by contributors are added regularly at http://sweethome3d.sourceforge.net/importModels.html .
OBJ is a wavefront text format described at http://download.java.net/media/java3d/javadoc/1.5.1/com/sun/j3d/loaders/objectfile/ObjectFile.html . You may create this kind of files using Art of Illusion http://www.artofillusion.org/ , a 3D modeler hosted at sourceforge.net.