78th way

There are a lot of arbitrage opportunities in the finance world as discussed below but if economics and finance talk bores you to death, there's sports arbitrage. Different online bookmakers may offer different odds on a sporting event. By taking the better odds at each bookie you could come out ahead irrespective of the actual outcome of the event. If you're really clever you could make a program to do this for you. Why don't the bookies adjust their odds based on what their competitors are offering? Because they have to balance their own books. Their prices are decided based on how their own punters are betting.