Laszlo Hanyecz -- the same man who bought two pizzas for 10,000 BTC -- publishes the first working GPU mining code for Bitcoin, proving that graphics cards could mine vastly more efficiently than CPUs. The release triggered an arms race that saw Bitcoin's hashrate explode by orders of magnitude. GPU mining democratized and intensified competition simultaneously, as anyone with a gaming PC could now mine far more effectively. This marked the beginning of the end for CPU mining and foreshadowed the eventual transition to dedicated ASIC hardware that would transform Bitcoin mining into an industrial-scale operation.
BitcoinTalk: GPU Mining Thread