When shopping for plane tickets, we won't settle for anything but the cheapest deal . We've got our usual lineup of fare-finding sites to check, yet a nagging feeling always leaves us wondering if we really found the lowest price possible. Is there any way to know for sure?

Yes, there is. ITA Matrix is probably the most efficient flight-finding site on the Internet, and yet you probably haven't heard of it.

The ultra-smart software is what powers both KAYAK and Google Flight Search . Developed by MIT computer scientists , it covers every special need you could possibly have when looking for a flight. You can find fares for exact dates or see a calendar of prices. You can book a round-trip ticket that arrives and departs from two different cities . You can even change your "sales city" in hopes of getting a cheaper ticket (aka the greatest travel hack ever invented ).

For hard-to-find deals, it's crucial to add a catchall site like this to your usual flight-finding lineup. As travel expert Matthew Kepnes explains in his new book, How to Travel the World on $50 a Day :

Whenever most Americans do a web search for airline tickets, they search the big three: Expedia, Travelocity, or Orbitz. People make a big mistake by only doing that... Many sites don't list budget carriers because they don't want to pay a booking commission. It's important to check multiple booking sites because all websites have their weaknesses and do not include every airline.

You can't book directly through ITA Matrix -- but you can use it to zero in on your dream ticket, then head to that airline's site or hand things over to a travel agent (the site gives you the booking and fare codes that agents need to find the same deal).

Then pack your bags , pour the cocktails , and enjoy the (cheap) flight!

For more budget travel hacks from Matthew Kepnes, visit the Nomadic Matt website or check out his book, How To Travel The World on $50 a Day .