Copied from WikiTravel.org which had a wealth of info on the topic :

http://wikitravel.org/en/Discount_ai...mparison_sites

Low cost airline comparison sites

These meta search engines look through many of the websites above and compare prices on one page.

* Volagratis.com Cheap flights and discount airline tickets search engine within and to Europe and worldwide.

* AirlineTickets.de Search & price comparison engine for very low airfares within and to Europe and worldwide.

* FlyCheapo.com Low-cost airline route search and news.

* Flylc.com European discount flights search engine.

* FlyLowCostAirlines.org Search engine of low-cost airlines' flights.

* TheLowcostAirlinesBlog.com lists the latest offers from discount airlines.

* Kayak Popular flight search engine which includes some European low-cost airlines.

* OptiFly.com Flight Search Engine and Visualization Tool for optimizing your trip using a combination of low cost carriers and major airlines.

* Momondo.com Low-cost airline search engine.

* Skyscanner.net Search engine that started out covering pan-European destinations and expanding to other countries around the world. Most discount airlines are covered, and the most useful feature is a monthly bar chart that allows you to compare the cheapest dates to travel on a calendar. The second most useful feature is the ability to define start- and destination by country instead of by city, with summary of best fares found for each city, if maximum flexibility and lowest fares are desired.

* WiseTravelSearch.co.uk This sites helps you search and compare the top online travel agents (including Kayak and Skyscanner) for low-cost flights.

* Travelmarket.com Search engine with agencies, low cost carriers and regular carriers.

* CheapOnlineFlights.com Compare scheduled, charter and lowcost airlines from the UK.

* Viviro.com Compares airfares from tradtional and lowcost airlines in addition to travel agencies. No cached prices