How can I measure the total hashing power of the Bitcoin network?
Network hashrate calculated using formula: H ~= h / t , where t is time that took to find X number of blocks and h is approximate number of hashes it should have taken to solve X number of blocks, h = X * (D * 2**256 / (0xffff * 2**208)) Wiki:Difficulty

Bitcoin network hashrate stats available at bitcoinwatch.com and bitcoincharts.com .