This is a very common request and need. People receive a DWG file and do not have AutoCAD to open the file. There are more DWG viewers out there than ants at a picnic, but there are only a two that are actually 100% DWG from Autodesk and totally free.
Two Great Free 2D & 3D DWG Viewing Options:
Autodesk DWG TrueView (Windows Desktop):
You can download the DWG TrueView. DWG TrueView is under the hood the the same exact code as AutoCAD. You can view, measure, plot with plot styles, and also convert files. It is a full Windows desktop application that you must download and install.
Get the free Autodesk DWG TrueView http://www.autodesk.com/dwgtrueview
AutoCAD 360 (Cloud Based):
This is where the power of the cloud is great. With AutoCAD 360 there is nothing to install on you desktop, just go to https://www.autocad360.com/ and then view, markup, and collaborate on the DWG file. You can control the people you invite to view it can do in the DWG and even save it as other file types or versions of DWG. You can even use the mobile version apps on Apple and Android platforms and view and markup your DWG files, even offline when you are in the field.
Check out AutoCAD 360 https://www.autocad360.com/