Competition heated up in 2012, as miners pulled together multiple GPU’s into mining rigs to generate more hashing power. Pools continued to form, with some early ones closing down. The big date this year was November 28th, known as bitcoin halving day. On this day, commencing block 210000, the reward for mining a bitcoin block changed from 50btc to 25btc. The price at the end of the year was about $13/bitcoin.
This page gives information extracted from the blockchain showing the best bitcoin mining pools of these years.