Since 2011, Draftsight is typically the top choice for free Linux/Mac/Windows AutoCAD 2D-compatible CAD users.
If you prefer libre software, you can consider QCAD or LibreCAD, then use the free Teigha converter mentioned below as a first/last step to convert to/from DWG <–> DXF.
I didn’t include FreeCAD here as it’s mainly for 3D / Solidworks (and also cannot open DWG at this time).
|License||free, NOT libre||GPLv3||GPLv2|
|Registration||YES install AND periodic REregistration||NO||NO|
||source or binary||source or PPA|
|DWG||YES: through R2017||free Teigha DXF-DWG converter||free Teigha DXF-DWG converter|
|DXF||YES: through R2017||yes||yes|
Draftsight: “easy” 2-D CAD
Draftsight is free and typically “just works” with most DWG / DXF someone sends you. But DraftSight is non-libre.
QCAD: GPLv3 2-D CAD
For using DWG/DXF, QCAD requires the non-libre, non-free DWG package (or use the free, non-libre Teigha DXF - DWG converter).
LibreCAD: GPLv2 2-D CAD
LibreCAD can read some DWG.
Free conversion DWG to DXF
Free, non-libre Teigha File Converter supports through R2017 DWG/DXF (which is using the Autocad 2013 AC1027 DWG format).
Teigha File Converter may be used to convert from DXF to DWG en masse as a first/last step. The OpenDesignAlliance is the same organization where QCad gets its non-free non-libre converter from.