Being publicly traded allows companies to fund expansions, compensate employees and bring attention to their products and services. It also places obligations on companies to be transparent, produce consistent returns and serve a broader set of investors.

Many of the most prominent publicly traded companies have familiar business models. They operate stores, provide financial services, produce and distribute energy, and drive innovation in technology, medicine and other fields. There are others, however, that operate unique, even bizarre, businesses, with few or even no publicly traded competitors.

Click here to see America’s Most Unusual Public Companies

Unusual companies operate niche businesses because they perceive opportunities for growth and — in the long-run — substantial profit. Oftentimes, this includes providing products and services in an underdeveloped sector of an established industry. For example, Clean Energy Fuels operates natural gas fueling stations throughout the United States and Canada, a unique business within the massive energy industry. GW Pharmaceuticals is the only publicly traded pharmaceutical company focusing on cannabinoid-based medications.

Other companies operate businesses that are usually privately held and closely guarded. Madison Square Garden owns NBA, NHL, WNBA and other sports franchises, as well as the television networks that broadcast most of their games. It also owns the arena in which the teams play. Similarly, Rick’s Cabaret operates adult clubs, venues that are typically privately owned.

Still, there are some businesses that are truly one of a kind. RPX Corp. specializes in pooling customer funds to buy patents, helping to protect its subscribers from aggressive patent litigation. Odyssey Marine searches the ocean floor for shipwrecks, acting as a publicly traded treasure hunter.

Unique companies can also range wildly in size. World Wrestling Entertainment operates a global wrestling empire watched by hundreds of millions of viewers worldwide. It has a market capitalization of approximately $1.3 billion. Madison Square Garden, with a market capitalization of $3.8 billion, is even larger. Premier Exhibitions, which operates exhibitions such as Bodies: The Exhibition and Titanic: The Experience, is considerably smaller than both — with a market capitalization of only $43 million.

These are the 10 most unusual public companies.

1. Clean Energy Fuels Corp.
> Business: Gas station
> Market cap: $812 million

Clean Energy Fuels Corp. (NASDAQ: CLNE) is not your run-of-the-mill gas station company. Rather than diesel or gasoline, the company’s gas stations only provide natural gas transportation fuels. The company aims to be the destination for a nationwide fleet of natural gas and compressed natural gas-run trucks. Clean Energy has close ties to billionaire and energy guru T. Boone Pickens, who serves on its board of directors and owns 24% of the company. It claims to deliver more natural gas transportation fuel than any other company in the United States. Clean Energy also claims that it fuels more than 35,000 vehicles each day at approximately 500 fueling stations in the U.S. and Canada. The company’s top line has grown, reaching $352 million in 2013. Analysts expect sales will grow to almost $500 million by the end of 2015.

2. GW Pharmaceuticals
> Business: Biotech
> Market cap: $1.1 billion

With marijuana now legalized in two states and decriminalized or medically available in many more, this is a hot area. While U.K.-based GW Pharmaceuticals PLC (NASDAQ: GWPH) does not provide what is commonly referred to as medical marijuana, its products include cannabis ingredients. GW can therefore be seen as a medical marijuana play. More accurately, though, the U.S.-listed company is a biotech outfit developing cannabinoid medicines. Its Sativex drug, which targets multiple sclerosis spasticity — stiffness and involuntary muscle spasms — has already been launched in 11 countries and approved in 13 more, although not yet in the United States. The company is also looking to target cancer pain with Sativex, and its drug pipeline includes Epidiolex, a potential treatment for childhood epilepsy.

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3. World Wrestling Entertainment
> Business: Pro wrestling
> Market cap: $1.3 billion

Vince McMahon has built World Wrestling Entertainment Inc. (NYSE: WWE) into global wrestling empire over the years. The company recently launched the WWE Network. The network, which had 667,000 as of April, seems well on its way to reaching its stated goal of 1 million subscribers by the end of this year. Despite some concerns about a shift to online viewing and away from live events and pay-per-view, WWE has managed to steadily grow its revenues. Sales were $508 million in 2013, versus $478 million in 2010. The WWE Network is expected to grow revenues to $581 million in 2014 and to nearly $800 million in 2015. In addition to the network, WWE also hosts Wrestlemania — the professional wrestling’s equivalent of the Super Bowl. Its investors currently receive a 2.8% dividend yield.

4. RPX
> Business: Patent risk management solutions
> Market cap: $862 million

RPX Corp. (NASDAQ: RPXC) provides patent risk management solutions. This means the company acquires patents for the express purpose of protecting its customers, who pay an annual subscription fee, from patent litigation. RPX was founded in 2008 and has been publicly traded since 2011. In addition to defensive buying, the company advises clients on patent acquisition, provides market research and even litigation insurance. Sales have risen consistently in recent years, up from $33 million in 2009 to $238 million in 2013. The handful of analysts covering RPX see sales growing to more than $292 million by the end of 2015.

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5. Collectors Universe
> Business: Collectible evaluation
> Market cap: $199 million

Collectors Universe Inc. (NASDAQ: CLCT) is unique to the field of sports collectibles, serving as a grader and authentication service for collectors of sports cards, autographs, memorabilia and coins. It claims to have certified more than 50 million collectibles to date. Revenue in its latest fiscal year totaled $49 million, and net income was $5.73 million. Collectors Universe has competition in sports card grading and in other aspects of its business, but it is unique in that it is publicly traded. In the company’s most recent nine months, its service revenue increased by 26% from the same period last year to nearly $44 million, a company record. Currently, the stock has a dividend yield north of 5%. The company is very small, with a market capitalization of less than $200 million.

6. Destination Maternity
> Business: Maternity clothing
> Market cap: $337 million

While many mainstream retail apparel companies offer maternity clothing, Destination Maternity Corp. (NASDAQ: DEST) is the only publicly traded niche retailer that specializes in clothing for expecting moms. It operates more than 1,900 retail locations under the brand names of Motherhood Maternity, A Pea in the Pod and Destination Maternity. It also sells online and through Sears, Kmart, and Kohl’s. The company’s shares have backed off recently, raising the dividend yield to 3.3%. Its 2013 revenues were $540 million, down slightly from the year prior. However, the company’s net profit was slightly higher in 2013, totaling almost $24 million.

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7. Madison Square Garden
> Business: Sports and entertainment venues
> Market cap: $3.8 billion

Madison Square Garden Inc. (NASDAQ: MSG) is the only publicly traded company whose main business is professional sports franchises. That is in addition to owning and operating Madison Square Garden, the self-proclaimed World’s Most Famous Arena. The company was spun off from Cablevision in 2010, and Cablevision CEO James Dolan still serves as MSG Inc. executive chairman. In its fiscal year ending in June 2013, the company’s sports division, which operates the NBA’s New York Knicks and the NHL’s New York Rangers, reported $470 million in revenue. The revenue of the media division, which operates MSG Network, was $678 million. Last month, MSG sold the cable music channel Fuse Network to SiTV Media for $226 million and a 15% equity stake in SiTV.

8. Odyssey Marine Exploration
> Business: Shipwreck exploration
> Market cap: $171 million

Odyssey Marine Exploration Inc. (NASDAQ: OMEX) specializes in the exploration and recovery of deep-ocean shipwrecks. It then sells the recovered cargo to the extent allowed by law. The company has noted in its annual report that it faces the risk of governments not recognizing its ownership of the recovered cargo. Odyssey was recently involved in a dispute with the government of Spain over an estimated $500 million worth shipwreck, dubbed the “Black Swan,” found off the coast of Florida. Odyssey’s share price tends to move higher when it finds major wrecks or is paid for wreck bounty.

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9. Premier Exhibitions
> Business: Museum exhibitions
> Market cap: $43.2 million

Premier Exhibitions Inc. (NASDAQ: PRXI) specializes in producing unique, museum-quality exhibitions, such as Titanic, Bodies, Real Pirates, Pompeii, King Tut, Cleopatra and Extreme Dinosaurs. The exhibits travel and are displayed in museums and other venues around the world. The company’s revenues have been sporadic, but reached nearly $39.5 million in its fiscal year 2013, up from $31.7 million in the previous fiscal year. Despite the revenue increase, the stock trades at around 85 cents a share. The company recently received a warning that it was out of compliance with Nasdaq’s requirement of a $1 per share minimum bid price and could be delisted if it does not comply before October 21.

10. Rick’s Cabaret International
> Business: Gentlemen’s clubs
> Market cap: $100 million

Rick’s Cabaret International Inc. (NASDAQ: RICK) is the only publicly traded company with a core business of owning and managing adult clubs. In addition to owning and operating 43 adult entertainment clubs, the company also owns adult entertainment websites and two trade publications, Exotic Dancer and StorErotica. The company has made a number acquisitions in recent years, greatly increasing its number of clubs. This includes acquiring 11 Jaguars gentlemen’s clubs in one deal in 2012, as well as numerous other individual transactions. In 2013, Rick’s generated $112 million in revenue. In March, it acquired a large exotic dancing club, Club O, in the Chicago suburbs, for more than $11 million.

Correction:  An earlier version of this article stated that RPX Corporation’s market capitalization was $85 million. In fact, its market capitalization is $862 million as of May 12, 2014.

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