ADAMANT has a post called Weighing the Web. It is a long post, well worth reading if you are into facts and figures flavored with some fun. Some calculations I liked are:

Last year’s power bill for the global internet was just $3 per capita- a bargain even by third world standards.

A statistically rough (one sigma) estimate might be 75-100 million servers @ ~350-550 watts each. Call it Forty Billion Watts or ~40 GW. Since silicon logic runs at three volts or so, and an Ampere is some ten to the eighteenth electrons a second, a straight forward calculation reveals that if the average chip runs at a Gigaherz, some 50 grams of electrons in motion make up the Internet. So as of today, cyberspace weighs less than two ounces.

By averaging on a global basis, it seems each person alive today has six watts of computational power at the disposal of their twenty watt brain.

Via Kottke.org