It\'s used on several websites which all seem to assume the reader knows what to do to have this type available, but I have no clue.
Example site, see the first comment:
You can get it from the COM component list. The assembly name is "Microsoft Internet Controls".
Add a COM reference to c:\windows\system\shdocvw.dll
This will create a .NET reference Interop.shdocvw.dll (or something similar) that includes the appropriate types.
It's not an assembly, it's a COM component. Project + Add Reference, Browse tab, select c:\windows\system32\shdocvw.dll. In Windows 7 pick shdocvw.tlb in the same directory instead. This generates the interop library for the COM component with the SHDocVw namespace. WebBrowser_V1 is one of the types you'll get from that.