Kayak uses several of the other popular ticket search engines as well as it's own to come up with cheaper tickets than anywhere else. I have saved hundreds and once even thousands of dollars using Kayak.

One of my favorite travel sites on the Internet. It takes the rest of the search sites and bundles them into a cozy package.

Attention Mile-Highers: The money you save planning your trip will pay for your bail when you get caught misbehaving on the plane...
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About Kayak.com:
Kayak is a travel search and booking engine. Unlike Expedia, Travelocity, and Orbitz — where several of its top execs hail from — the Kayak site does not sell travel directly.

When you request information about a flight or hotel, Kayak searches more than 100 major airline, hotel, and travel sites. From those it can access prices and itineraries on more than 550 airlines and 85,000 hotels — and then Kayak gives users the options of booking directly from the travel supplier of their choice by clicking on a supplier's direct URL, calling the travel provider's tollfree phone number, or booking via Orbitz or Onetravel.com.

The Kayak Advantage:

Kayak co-founder and CEO Steve Hafner says, “We created the site to meet the needs of today’s consumers who are frustrated at having to search multiple sites to find the best deal. With just one click, visitors at Kayak.com will be able to see prices and services in real-time.

"Kayak.com’s reach is so comprehensive that consumers will often find an itinerary on Kayak.com that they may not have found on their own. Not only does Kayak.com provide consumers with more travel options than any other site, but it also gives consumers the freedom to choose where to buy their travel."

This is a great deep-discount travel price comparison service. I use it primarily to source cheap airfares though they also provide comparisons for hotels, cars, cruises, and etc. Kayak scrapes the databases of 140+ travel sites and allows you to compare the findings. What's cool about the site is it's AJAX-heavy interface that allows you to see search results real-time as they are generated and also allows you to toggle your search parameters and see the results immediately without having to reload the page. They also have some cool customized services that will email you fare alerts to flagged destinations of your choice when the price drops to or below a certain benchmark.