I ran into this awesome open source tool for invoking web services. It looks at the wsdl file of the service and gives you a nice UI to enter the parameters that you wish to send over.
Tool name: Storm (previously named SoapBits)
The author’s page: http://geekswithblogs.net/Erik/archive/2008/05/10/122038.aspx
The codeplex page: http://www.codeplex.com/storm