.DWG stands for Drawing, and is a popular format used by many professional drafting software like Autocad. If you have received a .DWG file, but do not have an expensive software, you can use one of these Free .DWG viewers. All of these will open .DWG file for you, and some will even let you edit .DWG files. All these .DWG viewers are completely free, and work well.
DoubleCAD XT: Including this in a Free .DWG viewer category is almost insulting it, as it is a full fledged 2D Drafting tool like AutoCAD LT. It is completely free. It can indeed open your .DWG files, and you can also edit them. It can also import Google Skethup files, and do a lot more. Read more about this fantastic software here.
Autodesk DWG TrueView: This is a free DWG viewer from Autodesk. It is not just a DWG viewer, but also a CAD file conversion software for translating any AutoCAD design application or AutoCAD-based drawing file for compatibility.
Autodesk Design Review: This also comes from the house of Autodesk. It is an integrated, all-digital way to view, print, mark up, and compare versions of 2D and 3D drawings, maps, and models—without the original design-creation software.
eDrawings Viewer: This comes from house of SolidWorks. eDrawings Viewer allows convenient viewing of native AutoCAD files (DWG and DXFT) and native SolidWorks parts, assemblies, and drawings.
Bentley View: Bentley View is a free DWG, DGN and CAD File Viewer. It’s the best tool for projects that require a powerful viewing, measuring, and printing application for MicroStation, AutoCAD, or hybrid projects.
Varicad Viewer: Varicad Viewer is a scaled-down version of VariCAD. It can open STEP (3D), DWG (2D), DXF (2D), Varicad, and IGES (2D) files.
Free DWG Viewer: This free DWG viewer can view and measure DWG, DXF, and DWF files. It has XRef support to view layers & block attributes.
All these .DWG viewers work well, and some provide additional features like editing. Pick the one that you like.