Netremal is running an interesting series on web remote procedure calling methods.
Part one provides an introduction and demonstrates how clumsy it can be getting software to communicate via a webform.
Part two looks at designing and using a simple XML API to send and receive data.
Part three moves on to the first of three commonly used API structures, REpresentational State Transfer (REST), while
parts four and
five cover XML-RPC and SOAP respectively.