People wait at the check-in line at Baltimore Washington International Airport (Getty).

Boston: Providence and Manchester
In the old days, Boston had no low-fare service, so people flocked to where Southwest flew: Providence, Rhode Island, to the south; and Manchester up north in New Hampshire. Now, JetBlue dominates Boston, and Southwest flies there as well. That means there are fewer flights in the alternate airports, and the fare difference might not be as great as it once was. Even so, it’s worth a look.

Charlotte : Concord

You probably had no idea there was an alternate airport in Charlotte, and until recently there wasn’t. But Allegiant recently announced it from Orlando to Concord, on the northwest side of town, just in time for the holidays. There aren’t a lot of options, but if you’re going to or from Orlando, fares are probably going to be cheaper.

Chicago: Milwaukee and Rockford

If you need to go to Chicago, you know all about the two big players. Southwest dominates Midway, and pretty much everyone else goes to O’Hare. But if you’re heading to the far north side, Milwaukee, just over the border in Wisconsin, isn’t much further. When AirTran, Southwest, and Frontier were all fighting in that city, fares were much cheaper, but the advantage may not be as great anymore. If you’re heading west, you could try Rockford. Allegiant flies from there to Florida and the southwest. South-siders won’t have much luck. Allegiant pulled out of Gary just across the border in Indiana earlier this year and now that airport has no commercial service.

Houston:  Austin

Houston has two main airports with Southwest at Hobby on the south side and everyone else at Intercontinental up north. But a couple hours west in Austin airlines have been adding new service and fighting each other. Allegiant, in a rare move to a big-city airport, is flying to Vegas. And several airlines are fighting it out to the Bay Area. Next summer? Look for nonstop to London on British Airways.

Los Angeles: Burbank, Long Beach, Ontario, Orange County, Palm Springs

There is no shortage of alternate airports in the L.A. area. Burbank and Ontario are in the San Fernando Valley to the north and the Inland Empire to the east, respectively. Both have service from Southwest and other airlines to their hubs, though you aren’t likely to find significant bargains. Long Beach, however, is to the south and offers many regional flights throughout the west on JetBlue. Orange County is even further south with even more service, so it’s worth a look. And Palm Springs is further to the east with service from several low cost carriers, including WestJet in Canada.

Miami:  Ft. Lauderdale

Miami is one of nation’s most expensive airports for airlines to operate in, so low-cost carriers have shied away. Just up the road in Ft Lauderdale, however, you’ll find the home of low-cost carrier Spirit. Pretty much every other low-cost airline serves the airport as well. It’s absolutely worth a look.

New York City:  Islip, Stewart, and Westchester

New York surprisingly doesn’t have many good low-fare alternate airports. JFK is already the home of JetBlue, Spirit flies to LaGuardia, and Southwest has moved into both LaGuardia and Newark. Southwest reduced service to Islip on Long Island, but there may still be some deals. And Westchester and Stewart, both to the north, have service from several major airlines, including JetBlue. You might get lucky and find a deal, though I wouldn’t expect anything spectacular.

Orlando:  Sanford

On the northeast side of town, on the road to Daytona Beach, lies Sanford Airport. Sanford has lost some service over the years, but now it serves two main purposes. It is a major hub for Allegiant and it sees several low fare European charters. It’s further from Disney World, but the price might make it worth the trek.

Philadelphia: Trenton and Wilmington

For years, the closest thing Philly had to an alternate airport was Baltimore, and that’s still the case. But Frontier, a lower fare airline than Southwest has now found a home bracketing Philly’s population. In the north, it serves Trenton in New Jersey. In the south, it flies to Wilmington in Delaware. Both operations are small for now, but if this experiment works, there will be a lot more service.

Phoenix: Phoenix/Mesa

Phoenix Sky Harbor is the big dog in town, and low cost carrier Spirit recently moved its flights there. But Allegiant remains at Phoenix/Mesa, southeast of town, where it flies all throughout the western US. Not only might it be cheaper, but it may be the only nonstop option to many of those destinations.

San Francisco: Oakland, San Jose, and Sonoma

The best reason to choose an alternate airport in the Bay Area is to avoid SFO’s chronic delays, but you might find a decent fare as well. Southwest dominates Oakland, across the Bay, though JetBlue has some flights there still as well. San Jose in the south has more options available. But if you’re heading north and flying from the west coast, Sonoma is a little-known option served by Alaska’s regional subsidiary.

Seattle: Bellingham

Near the Canadian border, Bellingham is more often considered an alternate to Canadians escaping pricey Vancouver Airport, but it’s also an alternate for Seattle. Allegiant has frequent flights to the airport, and you’ll even find Alaska flying from there to Hawaii. It’s worth a look if you live to the north of Seattle.

Tampa: St. Pete

If you’re looking to go to Tampa, you could go across the bay and fly into St. Pete instead. Allegiant flies there from many small cities in the east, and Sunwing offers some some occasional Canadian service as well.

Washington, D.C.: Baltimore

Both National and Dulles are considered D.C. airports, and sometimes Baltimore is considered the same. But for those who aren’t in the know, it’s always worth looking at Baltimore. Fares have risen since the largest airline at the airport, Southwest, took over the second largest, AirTran. But there are still deals to be found compared to the other airports serving the city. It’s even connected to D.C. by rail.