What modules do I need to implement web services?
I've used the soapclient module in D6 for my cause with a SOAP server. It works by making SOAP calls and keeps things really simple. One method to instantiate the SOAP object and another to make the call. It has a good test page where you could enter the webservice end point, the mode like XML RPC etc. and other options such as the service method and service parameters and test the response which is quite good as you can test the service without any actual code. If you plan on implementing webservice with SOAP it is a nice module.