Low cost airfares to and within Europe

As the summer season approaches it is become quite obvious that for Americans travelling to the old continent 2007 will hold the record for the most expensive airline tickets. It is probably a mixture of high prices for oil, a low dollar and the fact that companies can still fill their planes with tickets that are being sold at a premium.

Who's had experience with low-cost airlines in Europe? What did you find the best websites to book them?

It's quite clear that summer tickets need to be bought early in the year if you want to get the best prices.