How do I consume REST as a client? 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?
Normally, I just roll my own. 

I use a combo of drupal_http_build_query and drupal_http_request to make the request.  I then use drupal_json_decode to parse JSON results, or 
simplexml_load_string to parse XML results.

When possible, I try to abstract out all of this into a PHP class as a singleton, which also includes static caching and/or caching via cache_set/cache_get for the actual requests.  I will then wrap up everything as a normal Drupal module.

Invariably, the service I am consuming will have something odd about it that precludes using a generic solution.  Also, I don't find the generic solutions any simpler than a custom one.