LowCostRoutes.com is a very simple, yet a powerfull tool for searching cheap flights.
It's main advantage is that it list all low cost airlines, regardless if they pay a booking fee to the booking agents or not. This way, they do list big airlines like EasyJet and RyanAir, that pay no fees to the booking agents. Along with the big airlines, small regional airlines are listed, so virtually every low cost flight is indexed on their website.
The search function is simple, allowing users to search flights from any city or airport to any other city / airport. It then display the full list of low cost flights from A to B, including direct links to the airlines. From this page, you can go and book directly from the airline's website, at the lowest price and with no commission.
A nice feature is airlines reviews. You can read the traveler's revirews about any low cost airline and then decide if you want to travel with it or not. You can also post your opinion about a jurney you had.
There is also a how to book cheap flights guide, giving tips and tricks on how to plan, search, book and use the low cost flights, in order to get the best prices. For example, they explain which are the best days to book a cheap flight.