Why use web services in Drupal?
Drupal as a service provider
Consuming Web Services in Drupal
Using FedEx Shipping Quotes module
Drupal and Multimedia Web Services
Using the Kaltura module and web service
Using the Media: Flickr module
Drupal Web Services the Easy Way: The Services Module
The Services module — what is it?
Creating a Services module and running a custom callback
Drupal, Spam, and Web Services