Online Travel Booking and Bidding Agencies - How to find cheapest city pair between two continents, Europe and Asia?

Most online flight booking tools are great when you know your departure and arrival city or country. I'm looking to find a booking tool that can suggest the best city to fly from and fly into when booking a long-haul international flight, for instance between Western Europe and Eastern Asia, with flexible travel dates (for example, throughout the summer). Is there any tool suited for this?

Thanks for suggestions!

http://matrix.itasoftware.com would allow you to plug in several cities on each end, allowing you to search quickly.