When you develop a Web Service application, either on the client side or on the server side, you need a tester tool. If you are on the client side, before spending too much time on your code, it's good to test whether the Web Services server is stable and have some general idea of the sample requests and responses. If you are on the server side, you definitely need a tester to prove your deliveries. Sometimes developers come up with an in-house GUI or web application tester and end up in a situation where there are more bugs in the tester than the application itself. Fortunately there are free, open-source and stable tools available nowadays. We don't need to reinvent the wheel.
Most of the time, Web Services are nothing more than SOAP/XML over HTTP. In the simplest cases, any tool that can send text over HTTP to an URL can be used to test Web Services. But you will find it a must-have to generate sample SOAP requests from any given WSDL. The famous XMLSpy provides this kind of feature, unfortunately with a hefty fee. The free version of soapUI by Eviware is my favorite.