Fileinfo.net Is a site that collects file extensions, categorizes them by application type, and which programs both Windows and Mac can open.

You can do direct searches, such as .dwg, or also by file type, for example "CAD".
This is the result you get when selecting the files that have the word CAD in their description.
- 1.
.2d
VersaCAD 2D Drawing
- 2.
.3d
Stereo CAD-3D Image File
- 3.
.3d2
Stereo CAD-3D 2.0 Image File
- 4.
.3d4
Stereo CAD-3D 2.0 Image File
- 5.
.adi
AutoCAD Device-Independent Binary Plotter File
- 6.
.asm
Solid Edge Assembly Document
- 7.
.bp2
AutoCAD Batch Plot File
- 8.
.bp3
AutoCAD Batch Plot File
- 9.
.bpl
AutoCAD Batch Plot File
- 10.
.catpart
CATIA V5 Part Document
- 11.
.cel
MicroStation Cell Library
- 12.
.cgm
Computer Graphics Metafile
- 13.
.com
Command File
- 14.
.des
Pro / DESKTOP CAD File
- 15.
.dft
Solid Edge Draft Document
- 16.
.dbb
FlashCAD Drawing Database
- 17.
.dgn
MicroStation Drawing File
- 18.
.dwg
AutoCAD Drawing Database
- 19.
.dxb
Drawing Exchange Binary
- 20.
.dxf
Drawing Exchange Format File
- 21.
.fbl
CADfix Command Level Log File
- 22.
.flx
FelixCAD Drawing
- 23.
.gsm
Graphic Description Language File
- 24.
.Yo soy
Inventor Assembly File
- 25.
.igs
IGES File
- 26.
.kmz
Google Earth Placemark File
- 27.
.lcf
ArchiCAD Library Container File
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- 28.
.lgh
HMI Historical Log File
- 29.
.lin
AutoCAD Linetype File
- 30.
.lp7
LP7 Digitally Signed File
- 31.
.mcd
Mathcad Document
- 32.
.mcs
Mathcad Image
- 33.
.mcx
MICRO CADAM-X / 6000 Model Data File
- 34.
.mde
ArchiCAD Educational Version Module File
- 35.
.mod
ArchiCAD Module
- 36.
.mvi
AutoCAD Movie Command File
- 37.
.pc6
PowerCADD 6 Drawing File
- 38.
.pc7
PowerCADD 7 Drawing File
- 39.
.pla
ArchiCAD Project Archive
- 40.
.pln
ArchiCAD Project File
- 41.
.plt
AutoCAD Plotter Document
- 42.
.pwd
Solid Edge Weldment Document
- 43.
.rcd
Arcade File
- 44.
.rgp
RealArcade Game Package
- 45.
.sdm
Spatial Data Modeling Language
- 46.
. Set
Settings File
- 47.
.sldasm
SolidWorks Assembly File
- 48.
.sddrw
SolidWorks Drawing File
- 49.
.sldprt
SolidWorks Part File
- 50.
.tcd
Technobox CAD Drawing
- 51.
.tcw
TurboCAD Drawing
- 52.
.u3d
Universal 3D File
- 53.
.vgd
Generic CADD VGA Driver
- 54.
.wrp
Geomagic 3D Wrap File
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Very useful resource to know for example that Mac and Windows applications can open a .dgn file
Mac OS
IMSI TurboCAD

Windows
Bentley View (free)
Bentley Systems MicroStation (commercial)
Cimmetry Systems AutoVue
Autodesk AutoCAD LDT
IMSI TurboCAD
ESRI ArcGIS