DXF References provided by Autodesk

• DXF R2018 Online Reference
• DXF R2014 Online Reference
• Download DXF References
• AutoCAD 2018 Developer Documentation

DXF/AutoCAD

• Between the Lines by Shaan Hurley - a treasure trove for all interested in DXF and other Autodesk related information
• AfraLISP() Learn AutoLISP for AutoCAD productivity
• AutoCAD DevBlog

Autodesk

• The Autodesk File Rev. 5 - Bits of History, Words of Experience - Insights into the History of Autodesk edited by the founder John Walker

Tools

• Autodesk Viewer a free online viewer for many file formats, but an Autodesk account (free registration) is required
• DWG TrueView an DXF/DWG offline viewer for Windows

AutoCAD alternatives

• BricsCAD
• ZWCAD
• GStarCAD