I am the main author pyNastran which is an open source Python-based interface to the Nastran file format. The software includes a Reader/Writer/Editor to the BDF, F06, OP2, and OP4 file formats. Additionally, there is a VTK-based GUI that supports Nastran, Cart3d, STL, Usm3D, Panair, S/HABP, and Cart3d. The secondary formats (e.g. other than Nastran) are a bit of an after-thought, but converting them to Nastran allows for common routines.