Blender lets you import, edit, and export OBJ files. So, you can edit an OBJ file in it. It provides some advanced features to manipulate the 3D model consists of an existing OBJ file. However ...

Sketchfab has built a huge library of 3D models. Some of them are available under a Creative Common license, others can be purchased. The startup now wants to help you do stuff with all this content.