When booking airfare online, most people think of the popular online aggregation sites. You know the ones: They have the fancy commercials, catchy jingles, and washed-up celebrity pitchmen. While those sites aren't inherently bad, there are a few well-documented problems with relying solely on these larger engines:

1. Many of the aggregation sites neglect to include smaller, budget carriers.
2. Larger airline companies may temporarily exclude or intentionally block these aggregation sites from fares.
3. Short-term specials or incentive sales aren't usually aggregated either. They're often only found by visiting the individual sites of the carriers.

Obviously, there isn't just one website capable of giving you the best deal every time. I wish it were that easy.

Many larger carriers make a significant amount of money off of the loyalty factor — meaning those individuals and companies who choose to fly the same airline every time for whatever reason. Because of this, they're not necessarily in a rush to make all their fare data open and available for the world to dissect.

In most cases, the cheapest fare will be found using a combination of sites depending on your specific travel plans. Here are some lesser-known websites you can add to your bag of tricks when searching for the lowest airfare available.

SkyScanner.net

SkyScanner is a traditional aggregation site. But it actively differentiates (and brands) itself, by including a wider variety of budget options than most larger sites of the same type.

Another helpful feature of SkyScanner is the ability to search for a whole month or even a whole year with just the click of a button. For example, you can choose December 2009 as your departure date and easily compare data for different days. If your travel plans are flexible, this is a convenient benefit.

In addition, you don't have to specify a specific airport, but rather just a city. Many other websites allow you to search flexible dates and close airports, but I haven't found any that make it as easy as SkyScanner. This convenience, and its access to some small budget sites, give it the nod of the larger sites in our book.

WhichBudget.com

WhichBudget is popular in international-traveling circles, and for good reason. Rather than pulling specific pricing information for flights, WhichBudget simply allows you to find budget airlines that fly a specific route.

For example, to plan our upcoming trip to Thailand, Courtney and I used WhichBudget to find budget airlines that flew various potential routes — Christchurch, NZ to Bangkok, Thailand for one. This is the type of flight that might not show up in the larger airfare search sites.

This service is especially valuable for places that play host to a variety of smaller, budget airlines, such as Europe and Southeast Asia.

BudgetFlightFinder.com

BudgetFlightFinder is an alternative site to WhichBudget. We've only been turned onto it recently, but I'm starting to favor it. Once again, it allows you to find budget airlines that fly a specific route.

Sometimes it's good to check multiple sites for information like this. For example, on a sample flight path of LAX to IND, WhichBudget shows only Southwest as a budget option. BudgetFlightFinder displays Frontier as the only option for the same path.

Once you've uncovered a budget carrier that flies your specific route, you can go to the individual website and double check the actual prices.

SeatGuru.com

SeatGuru is a unique site that contains the specific layouts of over 700+ different planes from various airlines. How is this useful information to you?

Well for each type of aircraft, they label the different seats as “good” (green), “some drawback” (yellow), or “bad” (red).

For example, because of the layout of the hull a specific row may have a considerable amount of extra legroom. Conversely, some rows may be a tighter fit or be close to the engines or infant seating. SeatGuru will show you in full color what to look out for.

They even break down where to find power outlets, overhead television locations, and, of course, bathrooms.

This may not directly save you time or money, but it can add value. Next time an agent asks you for your seating preference, you can tell them, “I'll take 12E, since it has a little extra leg room, a good view of a television, and nice accessibility to the exit after we land!”