View .dwg and .dgn files with this free application available for Windows, Linux and MacOS.
Teigha File Converter
Convert between .dwg and .dxf file formats with this Windows application. With wildcard support for input file specification, it allows .dxf files to be used as input.
Teigha Xtension SDK
Write plug-in modules for Teigha-based applications. Based on C++ and .NET this Software Development Kit (SDK) allows CAD users and third-party component developers to create custom objects and custom commands.
Specification for .dwg files
Use this open design document to specify the .dwg file format. It covers headers, entities, table entries, objects, raster images, object maps, and bookkeeping information.