When you create a default WCF Service Library in Visual Studio 2008 there is an additional Start Option to use the WcfTestClient.exe [/client:"WcfTestClient.exe"]. This start option can be found by right-clicking on the Project file in Solution Explorer > select Properties > select the Debug Tab.

This tool allows you to provide inputs to your service operations and see the responses back from the service.  The only problem is that this tool crashes with following exception:

Unhandled Exception: System.IO.FileLoadException: Could not load file or assembly 'svcutil, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. Strong name validation failed. (Exception from HRESULT: 0x8013141A)
File name: 'svcutil, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' ---> System.Security.SecurityException: Strong name validation failed. (Exception from HRESULT: 0x8013141A)
The Zone of the assembly that failed was:
MyComputer 

This is a result of a signing problem of the SvcUtil.exe tool

 you can disable strong name signing for the svcutil.exe by opening up a Visual Studio Command Prompt and running:

sn -Vr "%ProgramFiles%\Microsoft SDKs\Windows\v6.0A\Bin\SvcUtil.exe"

 Now you can debug your sevice and in the WCF Test Client.