Investing. Look to Machine Insight in Cambridge, and the work they've done. They're trying to train an AI system to be Warren Buffet in a box. You don't hear much about this trend, because people are making way too much money to talk about it openly.
I'll put it this way: if you can consistently beat the market by a few percentage points, you can be a billionaire.
I've heard anecdotes from multiple sources that there are groups of software engineers and algorithm designers buried deep inside all the major firms that are doing just this. I know those firms manage and create hundreds of billions of dollars and I've been told that the stiffest competition is in these tiny software components of the business.
But I don't mean to sound too snarky. Sorry about that. So go ahead and try to get a job at Machine Insight or another such firm to find out what they're all about.
http://www.machineinsight.com/
Typically this is not a question of "find the secret strategy and make billions!", but rather finding a good execution algorithm to break an order of $billions down into a series of smaller orders for a good average price, or finding and exploiting minute arbitrage opportunities, etc. Algorithmic trading is responsible for many billions of dollars of trades daily; I don't know offhand of particular people who have become billionaires off it, but there are definitely lots of folks who have made a lot of money.
You've taken my comment on a personal level in that you think I'm talking about an individual. I would form a company around the idea and target investing.
I'll put it this way: if you can consistently beat the market by a few percentage points, you can be a billionaire.