SubControllers are MVC Controllers that are also parameters to your action methods.  Incorporating their use in large systems allows for composition, dependency inversion, and separation of concerns.
Sometimes we want Controller actions to pass objects into SubControllers so that the SubControllers can do interesting things with them. 
For example, in a Product Controller we have the [...]

I should have known something was up when I caught Jeffrey Palermo (my boss at Headspring) browsing the ASP.NET MVC Preview 5 source.
We’re working on a enterprise web application using ASP.NET MVC.  That week one of our tasks was to create a rather complex page full of robust components that did intricate little things [...]