Most of the companies at the top of the Fortune 500 are common household names. They are massive retailers like Wal-Mart, energy giants like ExxonMobil, big automakers like Ford, and telecom titans like Verizon.

Discount Retailers Wal Mart Store Wmt

The biggest companies in the U.S. aren't all as ubiquitous as Walmart. Image source: The Motley Fool.

Yet some of the biggest corporations in the United States fly under the radar. Way under the radar. Here are eight relatively obscure companies that have a huge impact on the U.S. economy, with combined sales of more than $750 billion.

No. 8: Enterprise Products Partners (NYSE:EPD) isn't well known like the top oil producers, but it has an equally vital role in the U.S. energy infrastructure. Enterprise Products Partners operates more than 50,000 miles of natural gas and oil pipelines, getting energy products closer to consumers. Enterprise Products Partners also has some smaller energy businesses like natural gas storage.

Last year, these businesses generated $48 billion in revenue for Enterprise Products Partners. Recent turmoil in the energy markets is expected to reduce sales to a little less than $30 billion in 2015, but that still makes this one of the biggest companies in the country.

No. 7: Energy Transfer Equity (NYSE:ETE) is another little-known energy giant. It's in essentially the same business as Enterprise Products Partners. But Energy Transfer Equity is even bigger, with more than 70,000 miles of pipelines.

Last year, Energy Transfer Equity produced $56 billion in revenue. That's expected to fall to $47 billion in 2015. However, the pending acquisition of Williams Companies will probably lift Energy Transfer Equity's revenue back above the $50 billion mark in 2016.

No. 6: Dow Chemical (NYSE:DOW) is one of the largest chemical companies in the world. It produces huge quantities of plastics, performance chemicals, agricultural products, and more.

Dow Chemical generated revenue of $58 billion in 2014. Pricing pressure and the impact of the strong dollar have affected its revenue in 2015, but Dow Chemical is still expected to produce nearly $49 billion of revenue this year.

No. 5: Archer Daniels Midland (NYSE:ADM) may not be well-known to consumers, but it has a massive presence in the food supply chain. Archer Daniels Midland is a major processor of corn and oilseeds. It also provides various logistics services for agricultural products. Some of its oilseed products are also used in the energy industry.

Archer Daniels Midland produced $81 billion in revenue from these businesses last year. Like other commodity-exposed companies, it's expecting a revenue decline in 2015, but it will still probably eclipse $70 billion in revenue this year.

No. 4: Cardinal Health (NYSE:CAH) is one of three enormous companies that control most of the U.S. pharmaceutical distribution market. By buying drugs and medical devices in bulk, Cardinal Health can get big discounts from producers. This allows it to offer pharmacies, hospitals, and doctors better prices than they could get directly from drug companies, while still earning a profit.

Cardinal Health generated about $103 billion of revenue in its most recent fiscal year. With the rapid growth of healthcare spending in the U.S., analysts are expecting revenue to rise to $118 billion in 2015 and $126 billion in 2016. That should be enough to move it into the top 20 of the Fortune 500 within a year or two.

No. 3: Express Scripts (NASDAQ:ESRX) is the largest pharmacy benefits manager in the country. Whereas Cardinal Health is primarily a middleman between drug companies and pharmacies/hospitals, Express Scripts acts as a middleman between health insurance providers and pharmacies, helping to hold down the cost of prescription drugs.

This is a big business. Express Scripts generated $101 billion in revenue last year, and analysts expect it to grow gradually in the next few years.

No. 2: AmerisourceBergen (NYSE:ABC) is the second of the big three pharmaceutical wholesalers. Like Cardinal Health, AmerisourceBergen is benefiting from the rapid growth of prescription drug spending. It has the additional tailwind of a recently implemented partnership with top U.S. pharmacy chain Walgreens Boots Alliance.

In its recently ended 2015 fiscal year, AmerisourceBergen produced $136 billion of revenue, up from just $88 billion two years earlier. It isn't likely to maintain that torrid growth rate, but analysts still expect its revenue to reach $148 billion in fiscal 2016 and $158 billion in fiscal 2017. That will likely lift it into the top 10 of the Fortune 500.

No. 1: McKesson (NYSE:MCK) tops this list of the largest American companies you haven't heard of. Fittingly, it is the third-biggest pharmaceutical wholesaler. Like Cardinal Health and AmerisourceBergen, McKesson is profiting from booming prescription drug spending.

McKesson's revenue has leaped higher from $122 billion in fiscal 2013 to $179 billion in the 2015 fiscal year, which ended in March. Analysts expect it to surpass $200 billion in annual revenue by fiscal 2017. That will probably catapult McKesson into the No. 5 spot on the Fortune 500 by 2018. Not bad for a company that most Americans haven't heard of!

Adam Levine-Weinberg has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Express Scripts. The Motley Fool owns shares of ExxonMobil. The Motley Fool recommends Enterprise Products Partners, Ford, McKesson, and Verizon Communications. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.