Interessting tool to find flights…

Looking for a way to get from Essen (leaving not before 18:00) to Newcastle (arriving before 10:00 the next day) and going back from Newcastle (leaving not before 17:00) to Zürich (arriving before 10:00 the next day) Chis pointed me to a website that is very helpful for such tasks… (at least with the flying part of it): http://www.skyscanner.de

I wonder how hard it is to build a similar tool that takes further modes of transport (e.g. train and rental car) into account...