This is the list that reinterprets the organization's Top 500 high-performance ranking by focusing on energy-efficiency metrics, and the computers that are listed are all specific systems that are handling pretty super-duper, computation-intensive applications such as climate prediction or pharmaceutical research. The computer that rates No. 7 on the Top 500 list actually rates No. 1 on the Green500 ranking, which in my opinion isn't all that bad because it proves you don't have to sacrifice speed for green considerations.
One big take-away: IBM dominates the green list, not only claiming the top Green500 position but also rounding out the entire top 20 of that last. In fact, 39 of the top 50 "green" supercomputers were IBM-based technology. The average energy efficiency of the IBM systems on the list is 135.58 megaflops per watt.