Sepherosa Ziehau has made a number of improvements to TCP in DragonFly – specifically, nonblocking and blocking connect(2) performance. See each of his commits for statistics on how much this has reduced processor use under high load. He has also written up an extensive description of how all this TCP stuff works in DragonFly.
In similar news, he has a nginx patch that delivers a significant performance increase. It may go into nginx itself.