Price Comparison Websites

Yapta
Yapta says that it aims to help travellers with easy-to-use tools and information that assures that they get the best value from their travel spending.
Farecompare
FareCompare is an airline ticket comparison website. It allows you to quickly compare prices on thousands of airline flight options from multiple sources side-by-side.
Dohop
This is a price comparison search site and contains the flight schedules of more than 600 airlines and calculates all possible routes and connections for a given trip. It then checks airlines and travel agent websites for best fares for the trip and provides links to their site.
Momondo
Not an agent, this is a search site. It will check from a list of agents for the cheapest prices. It is financed through sponsored links and advertising. It collates the search results from over 700 other websites.
Kayak
This is a price comparison/search website which allows you to compare options, and when you find something that fits your budget and other requirements, you are given details on how to book. This may be direct with an airline, a travel agent or a tour operator. It can also be over the phone, or online.
Matrix from ITA Software
Matrix offers a comprehensive way to search for airfares for any itinerary in the world. It quickly and easily finds available flights with optimal fares. It is currently under development. One handy tool on this website is that you can search multiple destinations in one go, for example Heathrow to JFK, Las Vegas and LA by separating the destination airport codes by a ‘;’ You can then go through the pages of options without having to do a separate search for each route.
Travel Zoo
This is a slightly different comparison website. It also has a weekly newsletter in which companies pay to be included. This newsletter is very handy if you are very flexible in your trip. A pain as you have to use an email address to reach the site (I always make one up).
Amadeus
The site is owned and operated by Amadeus IT Group SA, which also provides GDS systems to airlines and other travel companies. They claim that you will find best fares, flight availability, flight timetables and flight status.

Travel Supermarket
This is the travel based search site for Moneysupermarket.com. It allows you to check departures from nearby airports as well as your preferred, and for up to three days either side of your chosen departure dates.