How do I consume REST as a client?

Question

I need to import on node save information from a third party REST server and map that data to an entity that references the node saved. There will be a field in the node that the user can input an id that is passed in the url of the REST call. The REST call will return an XML object that needs to be mapped to a new entity that is referenced to the node.

How is this done? Are there an examples of Drupal 7 modules that do this?