I wrote a blog a few years ago abbout booking cheap air tickets over here http://cybergypsy.eu/2009/11/02/cheap-flights-uncovered/.
Having just booked my Christmas hols I now have a few updates on that front so here we go...
Skyscanner.net has simply turned into an amazing tool.
I used to use tools like flightchecker at the wonderful moneysavingexpert.com, but skyscanner has started to include budget airlines into it's listings making it the perfect search tool.
the obvious is that you can search for an airfare from one place to another, but then you can open up another tool that allows you to compare dates for that route. This is amazing when you';re flexible, I saved over £300 by changing my dates by a few days, all of this at a glance.
It will also marry up different airlines including budget airlines, but be careful here. You don't want to choose a confusing interchange in Moscow to travel from Malaga to Madrid for a saving of a few Euro.
Anyhow here's a list of travel search tools to help you along!