The United States Department of Energy is the leading organization pursuing the development of practical exascale computers in the United States — no other US organization even comes close in terms of planning for the massive technology shifts we’ll need to get to exascale within a 20MW power budget by the end of this decade. And they are putting their money where their plans are, funding much of the needed R&D.
Much of the information drawn together at the Department’s many exascale workshops and programs are available on the Internet. A great place to start is the ASCR Program Documents page. Here you’ll find reports linking the computational requirements of critical science to their exascale roadmaps, as well as many other interesting documents. Great reading to get you up to speed.