HOW TO: Find cheap open-jaw flights

Just stumbled upon this site, it is exactly the sort of thing I was looking for. You type in the airport you want to depart from, click the nearby link and then select a radius, then click select all. It will then search all of these airports, you can do the same for the return airport. Then select calendar of lowest fares, enter the length (you can say 7-14), then click search. It will show a calendar of flights and their respective prices.

http://matrix.itasoftware.com/search.htm

There is also an Android and iPhone app called OnTheFly. Note that it doesn't seem to search Easyjet or Ryanair.

One example I found is flying from Dublin to LA and back is about £500 over Christmas, from London to LA is about £800 (even though it's the same flight and Dublin has a stop at London). Flights to Dublin on Ryanair are about £40 return.

Another one to try :-
http://www.skyscanner.net/

I tend to use sky scanner but I didn't think they allowed searching from multiple airports at once :/ once you find the flights you want on my link you can buy them on sky scanner.