One of the most incredible things I can think of using 3D for is the design, visualization, and marketing of products that have not yet been created!

One of the most incredible things I can think of using 3D for is the design, visualization, and marketing of products that have not yet been created!  For instance, if I have an idea for a cool widget, but I don't have the cold hard cash to create one, I could have the widget modeled in 3D, create an advertisement showing the widget in use by consumers in a video using the widget's 3D model, and offer the widget for sale on a website with various views and perspectives, and evaluate the demand of the widget based on website sales, calls, and various other inquiries.  

For those early adopters, I could 3D print the cool widget, and charge a premium, and if demand warranted, I could use the 3D model of the widget to create castings, molds and the tool and die necessary for mass production.  The interesting aspect of having a virtual item is that you can present it visually for promotional purposes, without the time, effort, and expense of actually creating a real widget, and save the expense in time and money until you have actually sold the widget!  Then 3D print or manufacture based on demand.

The blue doodad in the picture is a webcam mount I designed and had 3D printed for my robot after I had taken a CAD class at the local community college to prepare myself for writing a dissertation on something related to 3D printing.