If you are having ASP.NET web application that is using Report Viewer control, you should be having the following dlls on your machine:
The first dll, has to be added as a reference in your bin directory while the second one can reside in GAC.
You probably will be getting an error if you don't have the common dll on your machine, first, first check the GAC: C:\Windows\assembly
If you don't have it, then you need to install Report Viewer Redistributable package.
Here you have to note that the version of the dll will vary based on the installer version.
If you want version 10, you have to install VS 2010 release, here is the link:
If you want version 09.00.21022.08 of the dll, you need to install VS 2008 release:
If you want version 8 of the dll, you need to install VS 2005 release:
Hope this helps.