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:: Dime A C++ class library for reading, constructing, manipulating, and writing DXF file data. The name is an acronym for DXF Import, Manipulation, and Export library.
QCad is a small open-source CAD system that uses DXF. It runs on Linux. Review of QCAD.
StarOffice is reported to use DXF for something.
Open source DXF to OpenGL translator.
Sourceforge is hosting a DWG to DXF converter and viewer.
Sourceforge is hosting dxflib, an open-source C++ library for reading and writing (planned) AutoCAD (R) DXF files. It's at the moment very simple but already provides the functionality to read the basic entities of a DXF file.
Open CASCADE is an open source CAD tool, has DXF exchange.
ThanCAD is an open source CAD tool under development, and is expected to use DXF. No initial version is available yet.
The book 3D Graphics File Formats: A Programmer's Reference include DXF.
Some tools in the Web3D Repository support DXF.

Autodesk owns the DXF format. They provide documentation for the file format. AutoCAD 2000 and later releases are more strict and will not open files that contain DXF errors which may crop up in third party tools.
The DXF Format is documented by Autodesk.
Commercial Mentor to DXF bidirectional translator.
Sonnet's Sonnet-DXF to Gerber:

By toma