Simple CAD program
ZCAD is simple CAD program with dxf2000 file format support. Source code available here http://svn.shamangrad.net/zcad/trunk/
ThanCad is a 2dimensional cad, with limited 3d support and raster inclusion capability and emphasis in engineering. It is being written in Python and since it uses the Tkinter library, it is platform independent.
Autodesk Autocad Dxf FileFormat reading library in C# .NET.
Testsuite for libdwg
libdwg-testsuite is a test suite for libdwg. libdwg is "An open C library to directly read DWG files, a popular drawing format used in the CAD world." https://sourceforge.net/projects/libdwg/