You can check out the back story of Dixon’s first company, SiteAdvisor, here. It was built during a time when spyware was a booming business and browsers had few systems in place to combat bad actors. The company was acquired by McAfee in 2006. It's a great trip through the history of web security at the time.
Dixon next turned his attention to machine learning. He and his co-founders created Hunch, which worked to learn users’ tastes and recommend items they might enjoy. It was an early attempt to build the taste graph, a parallel to the social graph. It was acquired by eBay in 2011. Many of these techniques are now widely used across the biggest social networks in the world.
Dixon then moved into the world of venture capital. You can read more about the Crypto Fund he helps to lead and the new startup school a16Zz is launching to help educate a new generation of programmers and founders. Application are still open.
If you're interested in learning more about the background of Hashcash, which foreshadowed a lot of the ideas found in Bitcoin, there is some good info here.