I had the opportunity to write up a presentation about the Controller part of ASP.NET MVC. With Pro ASP.NET MVC 3 as my guide I whipped up this presentation and some sample code. Feel free to download/read/learn/look at it as you please.
One of my favorite things about controllers is the ability to return pretty much any type of content that you desire, files, xml, json, whatever. This makes controllers incredibly versatile and allows the developers to give the users power over what they get back (when they want to allow it).