gacutil.exe

Downloads Folder

.NET Download Cache
You can view the contents of the downloaded files by running

gacutil.exe /ldl 

.NET 4.0 Location

This is only included as part of Visual Studio

C:\Program Files (x86)\Microsoft SDKs\WIndows\v7.0A\bin\ 

.NET 3.5 Location

This was ony included as part of Visual Studio
This was only included in v1 and v1.1 of the .NET Framework

C:\Winnt\Microsoft.NET\Framework\v1.1.4322\ 
C:\Program Files\Microsoft Visual Studio.NET 2003\SDK\v1.1\bin\ 
C:\Program Files\Microsoft SDK\Windows\v6.OA\bin\