There have been so many awesome applications of 3D printing, I don't know which ones are the coolest, but this story is quite a display of what 3D printing excels most at. http://3dprint.com/8144/3d-printing-rocket/

Students at the University of Arizona were able to use 3D printing to print out major components of rockets in one piece, when ordinarily they would need to be constructed out of 7 parts. Additive manufacturing also enabled the students to create complex parts quite easily, when it would have been nearly impossible to do so with traditional forms of manufacturing. Check out the video below for these 3D printed rockets in action: