Blender is a open source 3D content creation suite available for all major operating systems under the GNU General Public License and available for free.  Many FlightGear developers use Blender, and there are a lot of articles on the FlightGear Wiki about using it.
Note: in order to import/export FlightGear's most common 3D format (AC3D, .ac) you must use Blender version 2.49b or older.
- Howto: Add smooth ("Ambient Occlusion") shadows in Blender
- Blender Ground Signs Tutorial
- Aircraft modeling tutorial (flightgear-specific)
- Aircraft modeling tutorial (non-flightgear-specific)
- Illuminate faces
- Make a helicopter
- Model Import and Export
- Modeling - FAQ
- Modeling - Getting Started
- Modeling - SketchUp
- Normals and Transparency Tutorial
- SketchUp to AC3D exporter