Galactic Center Saga by Gregory Benford. Im currently on the second book which is fantastic, the first was only so so but it seems to be improving as it goes along. Wiki Link http://en.wikipedia.org/wiki/Galactic_Center_Saga
Cities in Flight by James Blish pretty good saga set over a whole heap of years both on earth and in space wiki link http://en.wikipedia.org/wiki/Cities_in_Flight
I havent read over the whole lot of posts sorry if any have been mentioned already.