Network diagnostics for the end user

Are there any tools out there that we could give to our end users to help diagnose network problems? We get a lot of "the Internet is slow" support calls and it would be helpful if we had something that would run on the end user's computer and help characterize the problem. We have central monitoring system of course but that doesn't always give a complete picture, as the problem could always be on the end user's computer - slow hard drive, not enough memory, wrong name servers, etc.