In Postsingular, a mad scientist creates a race of nants -- nanites -- that digest the planet and turn it into a computational simulation of Earth, called Vearth. However, an autistic child memorizes a long string of numbers that poisons the nants and causes them to reverse themselves (luckily, they're engaged in reversible computation) and put the planet back. That's the setup.This is one of the most fun, strangest, most thought-provoking sf novels I've read, and it's fantastic to have it show up on the net, ready to be copied and shared. Link
Some time later, another race of benign nanos are released on the earth, the Orphids. Orphids are mezzoscale computers that organize themselves into an intelligent global network, tapping into every human brain and giving people access to outboard cognition facilities, so that anyone can drop out, tune in, and become hyperintelligent. The orphidnetters are haunted by spooks from a parallel dimension, who seek to prevent them from using the smarts of the orphidnet to develop interdimensional travel.