This wiki was in read-only mode for many years, but can now be edited again. A lot of information will need to be updated.
Blender
Blender is an open source modeler that can design and create BZFlag maps by using the plugin BZWTools. Alternatively, you can export objects as Wavefront OBJ's and use modeltool to convert them.
Blender is somewhat hard to learn. Many people prefer Wings3D because of this. If you're willing to take the plunge, however, Blender will pay off.
You can download Blender from Blender.org.