Whether you love it, or only sorta love it, pizza is big business. Over any two-week period, nearly two-thirds of Americans will eat some form of pizza, either frozen or at a restaurant, according to consumer-marketing research firm the NPD Group. Overall, pizza is the sixth-most popular dinner entree in the U.S. That's a lotta dough. From pizza trainers to eco-friendly pizzerias and big franchises, we serve up some of the most innovative entrepreneurs who have taken a bite of the giant pizza market over the years.

Pizza a Casa

Founder: Mark Bello
Year opened: 2010
Number of locations: 1
Signature pie: Mozzarella and basil pizza

Mark Bello (above) opened Pizza a Casa on New York City's Lower East Side, he says, after spending nearly five years as a "nomadic pizza-teacher-for-hire," sharing his pizza skills at various locations around the Big Apple. At Pizza a Casa, the customers don't buy pizzas. Instead, they make their own, and they each pay Bello $150 to teach them how. Since opening about a year ago, Bello says he's taught more than 2,000 students how to work their dough.

Naked Pizza

Founders (left to right) Randy Crochet, Robbie Vitrano, Brock Fillinger and Jeff Leach
Year opened: 2006
Number of locations: 14
Signature pie: The Superbiotic -- "prebiotic" fiber crust, mozzarella, herbed tomato sauce, mixed vegetables
Franchise startup costs: $275,000-$350,000

For the founders of Naked Pizza, the goal was to demonstrate that food can help fight obesity and chronic disease. To that end, they set out to create their version of the world's healthiest pizza. The crust is made of a blend of 10 grains and seeds, as well as prebiotics -- fiber that some claim improves digestion. The pies are topped with sauce made without transfats, butter, added sugar or high-fructose corn syrup; skim-milk mozzarella; and meats from animals raised without growth hormones or antibiotics. Also setting the business apart is its aggressive social media strategy which taps outlets such as Twitter and Facebook to communicate its vision to its customers.

Si's Pizzeria

Founder: Siler Chapman
Year opened: 2002
Known for: Award-winning dough tosser

One year after opening the first Si's Pizzeria in Fort Mill, S.C., Siler Chapman (above) was persuaded by a friend to compete in a dough-tossing contest. As Chapman found out, making pizzas is a lot different than competitive tossing. He was booed off the stage. He spent the next six months practicing and went on to win the freestyle acrobatic title at the U.S. Pizza Championships in Los Angeles. He also helped form the World Pizza Champions dough-tossing team, which took home three gold awards at the Pizza Olympics in Italy. In 2007, Chapman converted his three Si's Pizzerias to Donatos franchise restaurants. He's currently the franchisee of 18 Donatos units.

Pizza Fusion

Founders: Michael Gordon (left) and Vaughan Lazar (right)
Year opened: 2006
Number of locations: 10
Signature pie: Founder's Pie -- free-range chicken, kalamata olives, roasted red onion, tomato sauce, gorgonzola, mozzarella, provolone and parmesan cheese
Franchise startup cost: $360,000-$595,000

The concept behind Pizza Fusion is to combine pizza with environmentalism. That explains some of the unusual details found in Pizza Fusion restaurants, including containers made from corn starch, utensils made from potatoes and recycled blue jeans used as insulation. Then there's the pizza, made with organic, gluten-free, vegetarian and vegan toppings. "Being good to the environment and community is in our DNA," says co-founder and CEO Vaughan Lazar. "It's who we are."

Uno Chicago Grill

Founders: Ike Sewell and Ric Riccardo
Year opened: 1943
Number of locations: 155
Signature pie: The Numero Uno -- sausage, pepperoni, onions, peppers, mushrooms, chunky tomato sauce, and mozzarella and grated Romano cheeses
Franchise startup cost: $2 million

What people know today as Chicago deep-dish pizza is said to be the invention of Ike Sewell (above) and Ric Riccardo, who wanted a pizza that was more like a meal rather than just a snack. The men cooked up a recipe and opened the first Pizzeria Uno restaurant at the corner of Ohio Street and Wabash Avenue in Chicago. The concept gained in popularity. Their first restaurant outside of Chicago opened in Boston in 1979. Their first franchise opened the following year in Washington, D.C. While deep-dish pizza is still the standby, the brand now is Uno Chicago Grill. More than 100 items are on the menu.

Domino's

Founder: Tom Monaghan
Year opened: 1960
Number of locations: 9,300
Signature pie: Pepperoni
Franchise startup cost: $250,000-$375,000

Big dreams can sometimes be built on only a few hundred dollars. In 1960, Tom Monaghan (above) and his brother, James, took out a loan for $500 and bought "DomiNicks," a pizzeria in Ypsilanti, Mich. James left the business the following year, and Tom Monaghan renamed it Domino's Pizza Inc. in 1965. Buoyed by its "30 Minutes, or It's Free" delivery campaign of the 1980s and 1990s, Domino's opened franchise locations around the world. Monaghan retired in 1998 and sold Domino's to private investment firm Bain Capital for a reported $1 billion. In late 2009, Domino's reinvented its core pizza recipe to include a garlic-seasoned crust, a sweeter tomato sauce and flavored cheeses.

Toppers Pizza

Founder: Scott Gittrich
Year opened: 1991
Number of locations: 28
Signature food item: Topperstix -- cheese bread sticks
Franchisee startup cost: $286,628-$529,968

Former Domino's operations manager Scott Gittrich (above) says he turned down an opportunity in 1991 to acquire two failing Domino's franchise restaurants for $1 each. Instead, he opted to create his own pizza concept and opened the first Toppers Pizza in Champaign, Ill. Its quirky and edgy brand resonates with its target demographic of customers aged 18 to 34. It pizzas include the Mac 'N Cheese and the Hangover Helper -- a pie topped with Canadian bacon, onions, green peppers, potatoes, bacon bits, mozzarella and cheddar cheese.

Mystic Pizza

Founders: Stefanos and Fofi Zelepos
Year opened: 1973
Number of locations: 2
Signature pie: House Special -- the works

How many pizzerias have been featured in a Hollywood film? We can think of at least one: Mystic Pizza. First opened by the Zelepos family on Main Street in Mystic, Conn., this seaside establishment serves as the backdrop to the 1988 film of the same name, starring Julia Roberts. The film revolves around two sisters and their friend who -- you guessed it -- work at a pizza parlor in the town of Mystic. Experiencing the pizzeria-made-famous may be only one reason customers keep coming back. The sign out front says: "Mystic Pizza: A Slice of Heaven."

Ray's Pizza

Founder: Ralph Cuomo
Year opened: 1959
Number of Manhattan pizzerias with "Ray" in the name: 17

Original Ray's. Famous Ray's. Famous Original Ray's. Original Famous Ray's. Imitation can be the sincerest form of flattery, right? The first Ray's Pizza was opened on Prince Street in New York City by alleged mobster Ralph Cuomo. Five years later, Rosolino Mangano opened Original Ray's Pizza, and went on to start at least eight more. As the years wore on, it seemed as though a Ray's pizzeria had popped up on every corner in the city -- with variations on the name and numerous owners. But it's the Ray's on Prince Street that local pizza connoisseurs call "quintessentially New York."