Alternative Airlines
The company behind this website has a long track record in finding flights beyond the reach of major global airlines. This website is their consumer version. All reservations are confirmed immediately and the fares are quoted in your choice of Pounds Sterling, Euros or US Dollars. Prices are reported as being fully inclusive of pre payable taxes, airport/service fees and other obligatory charges, so, the fare you see is the fare you pay.

Fly Cheapo
This is a specialist guide to flying with low-cost airlines in Europe. They provide route and airline news and a flight search engine, which will show you which airline flies where.

Which Budget
This website is a budget airlines directory, and features dozens of airlines from around the World.

 

Jumble Fly
Due to its unique map search, this website allows you to select all airports you would consider flying from and to within a single search, rather than having to search each one separately. This might be useful if you live close to a number of airports, or are not tied to a particular destination airport.

Sky Scanner
This is probably one of the best known price comparison websites. They claim to be able to search over 670,000 routes flown by over 600 airlines. You are able to browse over a month or even a year in order to find a cheap price. If you are just after a cheap flight, you can even find the cheapest flight anywhere from your nearest airport.

OAG
This is a subscription website, but does have some free tools. Used by travel professionals and business travellers.