Here's a rundown of airlines flying within or to the U.S. which are offering their best fare deals only on their own sites.
Air New Zealand
Savings on routes from Los Angeles to London, and from Los Angeles and San Francisco to both Australia and New Zealand range from $50 to $200 or more, depending on season and route.
Alaska Airlines
Sign up for the airline's newsletter and you'll receive alerts about promo codes, which change every few weeks, with savings typically of 20 percent.
Allegiant Airlines
This airline, which mostly flies to/from Las Vegas, Orlando, and St. Petersburg only sells fares on its own site. Sign up for its newsletters.
American Airlines
AA recently launched DealFinder, which offers discounts typically of 20 to 25 percent on various routes. This tool works on most Windows computers but not Macs and requires that you download software.
Cathay Pacific Airways
This Hong Kong airline has frequent "deals of the month" at prices considerably lower than listed elsewhere, mostly from the U.S. to Asia, but also from New York to Vancouver. Savings range from $60 to $200 or more roundtrip.
Frontier Airlines
Frontier's weekend deals are only available on its own site. The airline occasionally sells non-weekend fares at discounts too.
JetBlue Airways
This carrier's fares are not listed on Orbitz, and although they do appear on Travelocity, Sidestep, Kayak, and Cheapair, among others, Airfarewatchdog frequently sees special deals available only on JetBlue's site. Recently, for example, JetBlue offered $30 off each roundtrip booked via a special link on its site for passengers buying fares with an American Express Card.
SAS
We've seen flights from Newark to Europe, but sometimes from other U.S. gateways, for hundreds less on SAS' site. Newark-Stockholm is a route that frequently goes on sale.
Skybus
Famous for its $10 one-way fares, this new entrant sells fares only on its own site.
Singapore Airlines
Not only does Singapore Airlines often have lower fares on its site, but we often find that seat availability is better (for example, you might get a nonstop flight on its site for less than a connecting flight on another site).
Southwest Airlines
Visit DING! On Southwest's site and sign up for an applet that alerts you, often once or twice a day, to short-lived sales which are only available to DING! users. Fares are typically $10-$30 lower than Southwest's other fares. Whether DING! or not, Southwest lists and sells its fares only on its own web site.
Spirit Airlines
This spunky low-cost carrier has almost-weekly sales with fares as low as a penny (plus tax, of course), for specific dates, sometimes far into the future. Typically the lowest fares are only available for four to six travel dates in each direction, but other deals are available over a broader travel period. Join Spirit's $9 Fare Club (trial memberships cost $9, or free if you apply for its branded credit card) and you'll have a wider choice of fares, all of which are bookable only on its site.
USA 3000
This East Coast airline frequently offers coupon codes, generally $10 off each one-way fare. USA 3000's fares are not listed on all OTAs but they do appear on Sidestep.
Virgin America
This new airline recently had a 10-percent-off sale for members of its frequent flyer program, available only on Virgin America's site.
WestJet
Canada's leading low-cost airline occasionally offers promo codes, such as a recent 20-percent-off insider deal, applicable even to already-low sale fares, on flights between Orlando and its Canadian destinations.