Three Californias is a trilogy by Kim Stanley Robinson written prior to the Mars cycle. Each of the three novels, The Gold Coast, the Wild Shore, and the Pacific Edge, posits a radically different future taking place in Orange County. I've just finished the Wild Shore, read the Gold Coast over spring break, and will be picking up the final one soon. The Gold Coast takes place in a total-sprawl landscape of designer drugs and party lifestyle, the Wild Shore takes place in a post-nuclear America quarantined by Japanese ships off the coast, and I'm not entirely sure what the other one is about.
While these novels don't quite stand up to the magnum opus that is Red Mars, and are not as dense in ideas and sheer brilliance, they are definitely worth checking out even if only as a prelude to his later work.

