Throughout my engineering studies there were jokes made by my professors (mostly mathematics professors) that referenced the fact that pure mathematicians strive to create mathematics with no practical application. Then a physicist or engineer comes along and finds a use for it.
I know that advancements have been developed for String Theory (maybe the only useful thing to come out of String Theory). But, in that vein, what are some of the most advanced or obscure mathematics that have real world, practical application to engineering, economics, computer science or such (especially if they are not well known)? And what branch of mathematics do they belong to?