No cash? Eat up, then clean up.

Some restaurants around the world let patrons pay what they can for a meal, or volunteer help in exchange for food.

Denise Cerreta remembers standing in her tiny sandwich shop, waiting for customers to walk through the front door. She had just left behind a successful acupuncture therapy practice after seven years to open a restaurant. But she had no restaurant experience, and her business was failing. On that same day, however, Ms. Cerreta had an idea that changed everything – she would stop charging set prices for the food she served. Instead, she would let people price their own food or allow customers to sweep the floors or wash dishes in exchange for a meal.

Five years later, Cerreta's idea continues to blossom at her restaurant, One World Café in Salt Lake City, and her vision to provide nutritious and affordable organic food to all has become a reality. Customers have the option of dropping money in a donation box on the counter, swiping their credit card to make a donation, or volunteering for a plate of food.

It sounds simple now, but it wasn't easy in the beginning, she says, recalling the early days. "I didn't even have a banner or a business card. I don't know how people found this place." Slowly, her restaurant garnered a niche in the Salt Lake City community through the generosity of others, such as the man who donated $200 so she could buy a banner for her restaurant.

Around the world, pay-what-you-can and volunteer restaurant concepts, such as the one adopted at the One World Café, are catching on from Denver to Vienna. And although each restaurant may have different reasons for allowing customers to pay whatever they can or to volunteer in exchange for a meal, they all share a desire to reach out to their communities through food.

The Terra Bite Lounge in Kirkland, Wash., serves coffee, smoothies, baked goods, and sandwiches with no price tags. Ervin Peretz, the owner, adopted a pay-what-you-can model after a discussion about the stigmatization of "getting and accepting free food," he says.

Customers "get a pleasant experience. There's a lot of trust and warmth in this process, and they can feel like they are doing something good.... They are supporting people that might not be able to pay," Mr. Peretz says.

In Vienna, the words "pay as you wish" and "all you can eat" line the windows of Der Wiener Deewan – a restaurant founded and run by Afzaal Deewan, a cook and cricket player from Pakistan, and Natalie Deewan, a former college student – that offers a buffet of five curry dishes at whatever price people see fit to pay.

At Dan's Restaurant in Biddeford, Maine, customers can also negotiate prices as well as portion size. Owner Danielle Ward recently started doing this after an elderly woman on a fixed income came in and said she couldn't afford most of the items on the menu. "People are really taking advantage of this service" to help them afford to eat out on a budget, Ms. Ward says. Although she considers the price negotiations to be temporary, she may make them permanent.

Cerreta isn't surprised that pay-what-you-can or volunteer concepts have caught on with other restaurants. In fact, through her foundation, One World Everybody Eats, she's happy to help other restaurants that want to adopt a similar model. Her website (http://oneworldeverybodyeats.org) lists information and tips for those who want to follow her example.

"Sometimes I have to pinch myself. Is this my life?" Cerreta says. "I'm looking forward to more people doing this and to help more people."

Cerreta's mentoring has already helped jump-start the SAME Café in Denver. When Brad and Libby Birky decided to open a pay-what-you-can restaurant in 2006, they had no restaurant experience – Brad, a former information-technology consultant, had only a few cooking classes from a local community college under his belt. But with Cerreta's guidance, the couple's restaurant gained a following in the community. Two years later, the cafe has received nonprofit status, and is bustling with new and regular customers who donate their time and money to sustain it.

The acronym in the restaurant's name stands for "So All May Eat." In a location once occupied by a coffee shop, the cafe is a 17-seat restaurant where organic soups, salads, and pizza are always on the menu. In a small kitchen, the Birkys prepare pizzas topped with spinach and feta cheese or pepperoni and portabella mushrooms and bake them in a convection oven.

They chop up ingredients for green chili and broccoli cheddar soup and toss Greek veggie pasta salad and couscous salad, which they then serve in a buffet line.

After volunteering in soup kitchens for years, the Birkys saw a need for their restaurant. "We wanted an outlet to get healthy food out there for folks who can't afford to buy fresh organic all the time," Brad says.

"A lot of people gravitate towards us [who] don't qualify for social services or don't make enough to afford the high living cost of Denver," Libby says. "There are people who rely on us, or otherwise they would eat ramen noodles."

One World Café, which has also qualified for nonprofit status, serves organic dishes, too. The restaurant also has its own garden plot for growing lettuce, herbs, and other fresh ingredients that go into the eatery's meals.

The menu changes daily – varying among different types of soup, quiche, dessert, and cold side dishes. On any one day, the menu might list items such as tofu and spinach rice soup, Asian chicken with sugar snap peas, and baked pear with caramelized nuts.

Does it really work to allow people to decide how much to pay for a restaurant's meal? Yes, say the owners. Customers rarely slink out of the SAME Café and One World Café without dropping some money in the donation box or offering to volunteer in exchange for a meal.

Although some people might be tempted to do otherwise, "People are good," Libby says, "and they want to do what's right and participate in a community and belong somewhere and be treated with respect."

The stories the Birkys and Cerreta share about their experiences shed light on their decisions to run pay-what-you-can restaurants.

Libby recalls when a customer came in and said he didn't have a lot of money and asked to volunteer later. "He said, 'You really trust me'? And I said, 'This is how this works. I believe that you'll come back,' " she says. "When you give people a chance to do the right thing, it totally works out."

For Cerreta, the beauty of a pay-what-you-can restaurant involves "the serendipity, the community, the random acts of kindness when you least expect it."