On a crisp October morning, more than 400 people have crowded into the café of a conference center in Burlington, Massachusetts. They are here for Innovation 2009, a conference for tech start-ups sponsored by the Mass Technology Leadership Council. There are no programs: No one knows what the sessions will be about or who will speak. But the topics are guaranteed to be relevant to the attendees, because they are about to come up with those subjects on the spot.

This is what's known as an unconference, an event programmed -- and sometimes arranged -- by attendees. For everyone who thinks the highlight of most conferences is the coffee break, unconferences offer a kinetic alternative to keynote fatigue. Canned speeches and passive audiences are out. Instead, attendees create the sessions on the fly, and intimate groups hash over subjects of their own choosing.

Unconferences have been around in one form or another since the 1980s. The format has recently gained currency as digital tools have made organizing easy and face-to-face interaction has grown more desirable for its relative rarity. "Something happened in Silicon Valley where people went so far in the direction of technology," says Mitch Joel, the president of the digital marketing firm Twist Image. "They wanted to bring back more of a '60s communal aspect, with people getting together in the spirit of democracy, instead of conferences organized from the top down, where everything is mapped out and marketed." Joel, who extols unconferences in his book Six Pixels of Separation, estimates several hundred unconferences take place each year, chiefly in the technology and marketing industries. "But there's not an industry I can think of where they wouldn't work," he says.

At Innovation 2009, attendees with ideas for sessions grab markers and scrawl their ideas on sheets of construction paper. As a harried facilitator charges around the room with a microphone, they stand and announce the subjects they want to discuss: monetizing social media; eliminating noncompete agreements; not getting booted by your VCs. Some attendees -- who include entrepreneurs, VCs, and technology gurus -- are topic experts and want to lead sessions. Other people are just seeking advice. "I'm completely unqualified to lead the session, but I'd love to get some input on creating the perfect advisory board," Bill Shander, CEO of Web design company Beehive Media, tells the room.

The last person has barely finished speaking before the session leaders -- about 25 percent of the group, which is typical -- stampede out to the hall and begin taping their topics onto an enormous paper grid tacked up on one wall. Meeting spaces around the building are plotted along the y-axis, session times along the x. As many as two dozen sessions run at the same time: Empty spaces will be filled in throughout the day as fertile conversations beget ideas for new sessions. Attendees roll with the punches. If no one shows up for a session, it doesn't happen. If too many turn up, the crowd could spontaneously decide to break into groups, or the leader could slot a second session later in the day.

As the papers go up on the wall, attendees crowd in, straining over one another's shoulders to see what they want to sample first. "I did not come in here planning to run a session," says Roger Matus, who sold his electronic-records company in May. "But I was talking to an investor friend I ran into in the lobby. I told him I'm doing some consulting on competitive differentiation. He said, 'You should run a session on that.' And I said, 'You're right! I should!' "

Although this unconference isn't free, many unconferences are, a fact reflected in their let's-build-a-stage-in-the-barn-and-put-on-a-show ethos. In those cases, what typically happens is a business leader finds herself craving interaction about a particular topic and reaches out to friends and associates. She sets up a wiki to manage logistics, arranges to use the company cafeteria after work, and -- presto! -- an unconference is born.

Some of the larger unconferences, such as Innovation 2009 -- an annual event with multiple sponsors -- have more formal underpinnings. Still, even a large unconference is a cheap date. Because the conference space was donated and no speakers were paid, Innovation 2009 was able to charge just a few hundred dollars per attendee. Fees for traditional conferences can reach 10 times that amount.

No matter their ambitions, most unconferences are governed by four principles of Zen-like simplicity:

• Whenever it starts is the right time. Whenever it is over, it is over.

• The people who come are the right people.

• Whatever happens is the only thing that could have happened.

• If you are not learning or contributing, it is your responsibility to find someplace where you are.

That last principle, commonly referred to as "the law of two feet," is especially important to sustaining energy and unpredictability. "Some session sounds like it's going to be cool, but it turns out it's not your thing. Move on to the next thing," says Kaliya Hamlin, a professional unconference facilitator who imposed something approaching order on the MassTLC event. "If you're leading a session, don't feel people are leaving because it sucks. They're choosing to be bumblebees and move among sessions and cross-pollinate."

Fourteen people are milling around a Ping-Pong table on the second floor of the conference building, waiting for Scott Friend, managing director at Bain Capital. Friend is almost 10 minutes late for his own session, or he would be if the "whenever it starts is the right time" rule didn't render the concept of late meaningless. His subject is selling to Fortune 500 companies, and Friend, co-founder of ProfitLogic, comes armed with enough advice for a solo show. But he ends up sharing the spotlight with someone else in the room, Peter Burrows, the walrus-mustached CIO of Adidas Group, who leavens decades of expertise with anecdotes and humor.

Friend's session was conceived in the spirit of mentorship: He wants to answer the questions of others. By contrast, James Reinhart and Chris Homer, co-founders of thredUP, convened their session to seek advice. A dozen attendees sit uncomfortably in low-slung beach chairs on a patio as Reinhart explains the business model for thredUP's online used-clothing exchange. The site launched six days earlier, and already the Today show, The New York Times, and MTV are calling. The founders want help deciding how best to exploit the PR tempest while they have infrastructure yet to build and funding yet to raise. The session is scheduled for an hour, but after 40 minutes, everyone has had his say. The group disbands early, and the founders leave with some useful advice (take every PR opportunity offered -- but be clear that you are rolling out the service in stages). Attendees wander back to the agenda wall to pick something else.

"Unconferences are a lot more engaging and receptive to networking than normal conferences," says Ben Rubin, co-founder and CTO of Zeo, which makes devices that help consumers track their sleep patterns. "You can shape the discussion in directions that interest you. And there are opportunities to mentor. At the average conference, I'm looking to get. Here, I can get and give at the same time." (The greatest testament to the event's success may have been the paucity of attendees observed making calls or checking e-mails.)

Still, the format doesn't work for everyone. "It was disorganized," says Rochelle Nemrow, an independent marketing consultant. "I wasted a lot of time going back and forth to check the wall because we didn't have something in our hands to tell us where a session would be. And I'm a foot and a half too short to even see the wall, so I had to wait until everyone else had gone." That's too bad, says Nemrow, because "the content was incredibly useful, and there was this great energy in the moment. If they can smooth out the logistics, it could be a beautiful end result."

For more on how unconferences work, visit openspaceworld.org. Kaliya Hamlin, who organized Innovation 2009, offers tips for planners and attendees on her blog at unconference.net.