In the past year, U.S. natural gas prices have fallen by half, and recently sat near a 10-year low. How cheap can gas get? Some say it may soon trade for next to nothing, and that has important -- though not-so-obvious -- implications for stock investors.

Oil and gas production is booming across North America. Companies that once engaged in hit-or-miss exploration can now find energy nearly every time they drill, thanks to new technologies. Oil is easily transported overseas, whereas natural gas is not, so world demand has kept crude prices high, but natural gas stockpiles have reached glut levels. Drillers usually respond to low prices by cutting production, and many are doing just that. But oil drilling tends to unlock plenty of natural gas, as does drilling for other lucrative liquids, such as propane, butane and ethane. The result: Natural gas production continues to rise, says the Department of Energy. With operators of gas pipelines and storage facilities reaching capacity, some may be forced to turn customers away by the end of the summer, causing gas prices to crash, says Jerry Swank of Swank Capital, a Dallas investment firm focused on energy.

Of course, that situation won't last forever. Firms will find new ways to transport and use natural gas, and prices will rebound. Already, U.S. electricity generation from natural gas has jumped 12 percent in the past year. At some point, investors will get a once-in-a-lifetime chance to buy shares of gas drillers like Chesapeake Energy cheaply and ride a rebound in prices, but we're not there yet, says Swank. Chesapeake declined to comment.

The U.S. energy boom is good news for the economy and investors. Analysts say it could spark a manufacturing renaissance, and some estimate that it will create 3 million or more new jobs by 2020, and add 2 to 3 percent to the gross domestic product. For now, investors should focus on firms that can readily turn cheap natural gas into profits. Chief among them are steelmakers because of their high need for energy. Nucor (NUE), the country's second-largest steelmaker, says it plans to use more natural gas in place of coking coal. Chemical makers can turn natural gas liquids into everything from fabrics to food packaging. Dow Chemical (DOW) is ramping up such activities by restarting an idle plant and building a new one.

Increased energy production should keep pipeline operators like Dominion Resources (D) busy, especially because it has assets close to shale regions. It is also working on ways to ship natural gas in liquid form to foreign markets, where prices are much higher than in the U.S. Finally, Canada's Agrium makes fertilizer and is benefiting from rising world food demand. Natural gas represents a key cost for the firm; its price gives Agrium an edge over rivals for years to come.