Nice Post on Controllers that extend an API
I just read Nathan's post on RESTful controllers:
REST: From theory to practice
I found this article helpful. One of the things about RESTful applications that always felt awkward was how to go beyond controllers that fronted a model object. I thought the examples in this post made sense as concrete examples of where you would want to create "subcontrollers" that provide an extended API and additionally functionality for a single model.