Kayak: Kayak.com is the best aggregator to use, since it offers the best array of search tools and the least complex search. Kayak offers a very easy to use interface, very much like Google's basic interface, and offers an array of tools to help you narrow your search results into a list of flights you desire. For example, Kayak offers a slider to narrow your flight results to flights that arrive at a certain time; Sidestep lacks this feature. Kayak is also in the process of releasing a flexible date search tool, that will let you search up to three days before or after your preferred departure or arrival date.

Sidestep: Sidestep.com is quite similar to Kayak, but I dislike it in that it doesn’t offer nearly enough options to filter results, and its search mechanism doesn’t search as many possibilities as Kayak’s for some, more complex routings. Sidestep offers most, but not all of the tools Kayak offers that help you analyze your results. But, Sidestep does offer its advantages, namely its toolbar. Sidestep offers a free downloadable toolbar that allows you to automatically compare prices when you search sites such as Kayak or Expedia, saving you time and money. For example, you can’t find JetBlue’s fares on Expedia, but if you use the Sidestep toolbar, you can search JetBlue’s fares as well, perhaps saving you some money.

Mobissimo: Mobissimo.com is very much like Kayak and Sidestep in that it searches multiple travel sites at once. But Mobissimo is different in that it offers superior international search capabilities. Mobissimo searches foreign airline sites other aggregators miss. As more and more international carriers begin to offer their lowest fares online, Mobissimo can give you an edge when it comes to finding international flights. Mobissimo also searches a wide swath of consolidators who sometimes have lower international fares than the airlines themselves.

Booking Buddy: I mention Booking Buddy for two reasons. First, Booking Buddy is listed next to most fares on the Airfare Watchdog as a location to search for flights. Also, Booking Buddy conducts an immense amount of online advertising on various travel sites, and many believe Booking Buddy is superior to Kayak or Sidestep. Booking Buddy is an affiliate site that offers a way to save a little time. Instead of going to airline Web sites, fare aggregators, or third-party sites individually and entering in your info, you need only do it once at Booking Buddy and you can get to nearly any airline Web site or aggregator with your information already having been entered. However, if you want to use a flexible date search tool or buy blind to really save money, then Booking Buddy is simply a waste of time for you.

Farecast: Farecast may smash some kayaks soon if all goes well. The Seattle startup not only lets users search multiple sites like Kayak or Sidestep does, but aims to predict whether fares will increase or decrease in the next week, telling searchers whether to book right away or wait a week. Currently the site is in beta and only lets users search for flights from either Seattle/Tacoma or Boston, and the site doesn’t search flights that arrive outside the United States. But, if all goes well, this site may let users search for flights from most major American cities by late 2006 or early 2007. The Seattle Post-Intelligencer reported on the launch. Another site, FlySpy plans to offer a similar service but has not offered a beta version yet.