David Slone was hard to miss walking into the Blackbird Parlour, a small cafe in Williamsburg. A man of considerable girth, with wild black hair and a bushy beard, he was wearing purple velvet pants, a long black trench coat (also velvet) and spit-shined black loafers. Around his neck he wore a silver necklace with an outsize Star of David.

Mr. Slone, who lives in a slovenly loft and says he gets “ornery when he’s hungry,” has one quality most New Yorkers lack: He likes having total strangers stay overnight. In the past three years, over 100 such visitors have spent at least one night, and occasionally more, in a small corner room in his apartment. A 39-year old actor with philosophy and law degrees from Cornell, Mr. Slone is not running a stealth hotel; the guests pay nothing. And most have gone away happy: “Great guy, great place,” wrote a visitor from Villeurbanne, France. “We spent three days surfing David’s place, which is the perfect spot to go and discover New York City.”

“Every day with him was a highlight,” wrote Maya Sternal & Kazoo, an older couple from Hamburg, Germany. “Thank you again.”

The positive reviews appeared on Mr. Slone’s profile on couchsurfing.com, the Web site of the Internet-based social networking organization that helps travelers find free lodging—usually in the form of a natty old couch—as they traipse across the globe.

There are over 400,000 members in the couch-surfing project, dispersed throughout 224 countries, according to the Web site. Becoming a member is as simple as going to the site and creating a user profile, filled with as much or as little information as desired. So far, about 1,500 New Yorkers have signed up.

While couch surfing might seem a natural fit for warm-and-fuzzy cities such as Seattle and Boulder, the idea of visitors from Europe and elsewhere laying their heads to rest on complete strangers’ pillows in New York City—even the tamed Bloomberg version—seems more like a VH1 reality show gone haywire than a functional social contract.

The couch-surfing Web site has several protocols and suggestions to protect both the hosts and the surfers. If a member wants to act as a host, there are three verification levels one may sign up for, the highest of which involves a $25 credit card check. And hosts are of course given a free hand to accept or reject interested surfers, after an exchange of e-mails and phone calls. And even then, members are not obligated to let people into their residences, and can limit their involvement to meeting potential surfers in public places for drinks and conversation.

Couch surfing is the brainchild of Casey Fenton, a 29-year-old college dropout and New Hampshire native, who took the Web site public in January 2004, after selling his dot-com company. On his own couch-surfing user profile, Mr. Fenton quotes Jack Kerouac’s On the Road: “… the only people for me are the mad ones, the ones who are mad to live, mad to talk, mad to be saved, desirous of everything at the same time, the ones who never yawn or say a commonplace thing …”

And he has a mission statement: “CouchSurfing seeks to internationally network people and places, create educational exchanges, raise collective consciousness, spread tolerance and facilitate cultural understanding.”

If that sounds like a 21st-century euphemism for hooking up, well, the site carries the disclaimer: “I understand CouchSurfing Is NOT A Dating Site. The purpose of the network is to foster cross-cultural connections; it is not to be used to romantically pursue other members.”

Meet Nicole Caldwell. The petite 26-year-old editor in chief of Playgirl magazine, Ms. Caldwell has hosted six or seven surfers in her tiny West Village studio apartment, sometimes throwing in a tour of local watering holes. She chose not to let her family know about her hosting, and, she noted, “All of my guy friends thought I was crazy.” Because of her prime location, she said, she was inundated with couch requests when she first signed up. She helped winnow the field down by cherry-picking attractive European men, which created, she admitted, “a sexually charged atmosphere.” To keep safe, she would have a friend at her apartment when her guests first arrived. But the chaperone wasn’t always needed: She admitted she hooked up with at least one of her surfers. And she enjoyed the adventure of the whole scene: “It’s a sense of hitchhiking without going anywhere,” she said. But Ms. Caldwell recently took a break after a pair of Spaniards overstayed their welcome.

Some travelers who surf New York’s couches end up with some local history as well. Matt Levy, a 27-year-old Flatbush resident with a handle-bar moustache who has hosted 30 to 40 surfers, runs a Manhattan-based tour guide company with his father and brother. They specialize in historical walking tours, for which Matt and his brother sometimes don period customs; Mr. Levy occasionally guides tours dressed as a 19th-century street tough. His experiences as a host have been positive. “It’s a great system,” he said. “All based on karma, all based on trust.”

Mr. Levy has surfed as well as been a host. Two years ago he and his girlfriend couch-surfed across South America. Last fall, he made it to Beijing, where he split his time between a bachelor pad in a kinetic working-class neighborhood and a luxurious, spacious apartment in the diplomatic district—all for free, of course.

Scrolling through user profiles of New York members, you’ll see that most people are on the south side of 30; Mr. Slone said he’s noticed a “hippy-dippy vibe” among couch surfers, and sees an overlap between the couch-surfing and Burning Man crowds. Which makes Paul Hsiao, a bespectacled and a soon-to-be 50-year-old computer repairman, something of a couch-surfing anomaly. A converted Baptist who is a great believer in the couch-surfing ethos, Mr. Hsiao was wearing a red beret and a $100-bill-patterned tie on a recent afternoon in his computer repair shop in Brooklyn’s Kensington neighborhood. Originally from Taiwan, he discovered the couch-surfing Web site from a Harvard student who was staying in his house. He said he has hosted about 10 surfers, and likes to perform magic for his guests (he belongs to two magicians’ associations.) He also likes to take them to coffee shops in Park Slope and, if he’s not busy with work, Manhattan. Mr. Hsiao is married and has two children in high school; his wife and kids don’t seem to object to his willingness to open the home to visitors.

Not all couch-surfing tales end smoothly. Tina Mancusi, a singer-songwriter with wavy brown hair and delicate facial features, once hosted a surfer who turned out to have fabricated her entire life story. She arrived at Ms. Mancusi’s one-bedroom Murray Hill fifth-floor walk-up claiming to be in her late 30’s, but her appearance suggested something closer to 50. The woman, whom Ms. Mancusi later learned was a photographer from Philadelphia, made herself at home, helping herself to a breakfast of scrambled eggs and sharing overly personal life stories. Later, when they visited an art gallery, the woman pretended that it was her work that was on exhibit. Ms. Mancusi made a run for it when her guest starting hitting on the men in attendance.

Strangely enough, however, most surfing experiences seem to go well. After almost 100 guests, for example, Mr. Slone said he had yet to have an awful situation. Yes, a drunk couch surfer did break his wardrobe, and others were messy. But the bigger problem among hosts is burnout: There are only so many mornings one can climb over hung-over Europeans on the way to the fridge before it gets old.

In late November, Ms. Mancusi decided to host a New Year’s Eve Party in her 600-square-foot apartment. She invited about 60 friends and, on a whim, posted a message about the bash on couchsurfing.com. Erin Fisher from Torrance, Calif., R.S.V.P.’d; so did John Pierre, from Kenya. The night of the party, between 200 and 300 people—including Germans, Australians, Brazilians, Japanese and even a few Canadians—showed up.

“Everyone,” said Ms. Mancusi, “was superfreaking fantastic.”