AutoDesk 2009 DXF Reference
 

http://images.autodesk.com/adsk/files/acad_dxf.pdf
AutoDesk created and maintains the DXF standard. This reference explains DXF file format and structure as well as all of the group codes and values.

Example DXF Files

• shapes.dxf.zip -- contains a variety of standard DXF shapes
• text.dxf.zip -- contains a number of text blocks set to different alignment properties and rotations (also, here's a PDF of what this text example looks like in AutoCAD for comparison: TEXT object example.pdf)
• floorplan.dxf.zip -- a sample architectural floorplan

Tomacorp
DXF Tool Summary

http://tomacorp.com/dxf.html
Nice summary of existing DXF import/export tools and libraries

Dime
C++ DXF import/export
library

http://www.coin3d.org/lib/dime/
Object oriented DXF import/export library. Well developed and functional. Only licensed as part of the Coin3D product for commercial use. Free for non-commercial use.

dxflib
C++ DXF import/export
library

http://www.ribbonsoft.com/dxflib.html/
Semi-object oriented simple dxf import/export library written in C++. Although it is written in an object oriented context, the actual import routines are mostly procedural. This library does not properly handle BLOCK and INSERT objects/entities. It is a relatively simple library. It is free for non-commercial use, but there is a licensing fee for commercial use.