Matrioshka Brains

From Wikipedia:

The possible uses of such an immense computational resource tax the imagination. One idea suggested by Charles Stross, in his novel Accelerando, would be to use it to run perfect simulations or "uploads" of human minds into virtual reality spaces supported by the Matrioshka brain. Stross even went so far as to suggest that a sufficiently godlike species utilizing enough raw processing power could launch attacks upon, and manipulate, the structure of the universe itself.
The theme of computers simulating human life is not new to the world of science fiction, forming central parts of the plots of such works as the Matrix film trilogy and the Star Trek: The Original Series episode "A Taste of Armageddon".

http://en.wikipedia.org/wiki/Matrioshka_brain