Airline passengers are getting dinged for everything from checking a bag to upgrading beyond the middle seat.

As carriers nickel-and-dime their customers, it’s hard to believe they would offer a cool new amenity at no cost. But that’s exactly what’s happening.

Several airlines with Wi-Fi-equipped airplanes are letting passengers try out the service for free.

Last Saturday, on her connecting flight from Salt Lake City to Washington, D.C., Pam Scott of Spokane, Wash., got her first chance to try in-flight Internet service. She surfed — for free — thanks to a promotion offered by Delta Air Lines and Aircell, the provider of the Gogo in-flight service. “Loving it,” Scott wrote in a message sent from the skies, “Nice to be in touch on such a long flight.”

Austin, Texas-based event planner Nichole Wright’s first taste of in-flight Wi-Fi was also free. She was on her way to New York City on business when a Delta flight attendant handed her a free pass. “I was thrilled,” says Wright, “and it worked very well; a huge time saver. I think I would pay for it in the future.”

Food blogger Alejandra Ramos would probably pay, too. Access to in-flight Wi-Fi was complimentary the day she flew with United Airlines from New York’s JFK airport to San Francisco. It was an extremely turbulent flight, so Ramos focused on e-mailing with her online friends. “I told them all about my nervousness and it was nice to have several dozen of them giving me their tips for staying calm while flying.”

So how can you cash in?

In some cases, you’ll need to be at the right gate at the right time, or on an airline’s targeted list of fliers. But there are current offers that will benefit everyone:

American Airlines
Through Saturday, Nov. 11, passengers on Wi-Fi equipped American planes (all Boeing 767-200s and some MD-80s) can use the service for free if they sign in with the code 2010LEXUSLS. The free pass comes courtesy, unsurprisingly, of Lexus.

AirTran Airways
From now through the end of the year — and perhaps longer — in-flight Wi-Fi is complimentary on AirTran flights on the competitive route between Baltimore and Boston. “We think free Wi-Fi gives AirTran the competitive advantage on that route,” says spokesperson Christopher White. All of the airline’s 138 airplanes are equipped for the service, and anyone who buys one session before Dec. 31 gets a free session they can use on another AirTran flight before January 31, 2010.

Delta Air Lines
The “First Timers are Free” campaign currently has no set end date, so anyone who signs up for a new Gogo account through Delta gets their first Wi-Fi session for free. Delta spokesperson Paul Skrbec says the airline is also providing promotional trial codes to customers waiting in some airport gate areas and sending targeted offers to some members of its SkyMiles frequent-flier program.

United Airlines
Through Nov. 15, in-flight service is complimentary on some United jets flying between New York’s JFK and San Francisco or Los Angeles The airline will begin charging when Aircell completes installing the Gogo service on all 13 planes flying the routes.

Virgin America
And in what is perhaps the most generous offer available, Google is sponsoring free Wi-Fi for everyone on all Virgin America flights between Nov. 10 and Jan. 15, 2010.

Searching for the sweet spot
“Equipping airplanes with Wi-Fi service is far from free,” says Doug Backelin, American Airlines’ manager of in-flight communications and technology.

Airlines are trying to figure out the right price. For some, the sweet spot for the new amenity is free — for now.

But how can airlines make money from free?

Glenn Fleishman, a freelance technology writer who has followed in-flight Wi-Fi since 2001, assumes some airlines are giving away the service now in an attempt to prime the pump. “This could be a clever strategy to get a lot of people hooked.”

Henry Harteveldt, a travel industry analyst at Forrester Research, agrees. “Speaking as someone who can’t imagine not taking advantage of Wi-Fi service on a Wi-Fi equipped plane, I can tell you that offering free trials will definitely help airlines increase the paid usage of this service” Harteveldt said the average take rate — the number of users signing up for the service — on a Wi-Fi equipped airplane is already between 15 percent and 30 percent, and that figure jumps significantly on long-haul flights and on weekdays.

The number of repeat users is also somewhat encouraging: an Aircell representative reports 31 percent of passengers who use the Gogo service each day have used it before.

Good, but not good enough
If you believe the results of a recent study conducted by the Wi-Fi Alliance in conjunction with Wakefield Research, the numbers should be better.

The survey found that frequent fliers and business travelers value in-flight Wi-Fi above meal service, free movies and other perks. The research also showed that 76 percent of the respondents said they would choose an airline based on the availability of in-flight Wi-Fi.

Jason Beahm, a San Francisco-area attorney, only flies Virgin America because the airline offers Wi-Fi (and power outlets) on all flights. “I’m able to get work done and catch up on news and e-mail, play chess, chat, and do anything else I would normally do on the computer.”

The race is on
“We’ve found that once people try the service they rarely fly again without it,” says Virgin America’s Abby Lunardini.

Perhaps. But Virgin America and other airlines must first get people to try the service. So far, it seems, not enough people have been willing to pay up.

That explains all these free offers, but as American’s Backelin points out, no airline can afford to keep giving away the service. In other words, these free samples won’t be unlimited.

On the other hand, you may not be looking at paying top price for in-flight Wi-Fi service. Like Lexus and Google, other advertisers and sponsors may be willing to occasionally pick up or subsidize the cost. And Forrester’s Harteveldt suspects that a year from now, we may find airlines offering Wi-Fi as a complimentary service to certain passengers.

While airlines aren’t spilling the beans about upcoming promotions, most say fresh offers and promotions are in the pipeline.

And don’t assume the price you pay today will be the same tomorrow. Southwest and Alaska are experimenting with various price points for the satellite-based Wi-Fi services they are slowly installing.

Last week, Aircell, which provides the Gogo in-flight Internet service to AirTran, American, Delta, United and Virgin America, lowered the price on its 30-day pass from $49.95 to a more reasonable $24.95.

It’s not free, but it’s a step in the right direction.