The Swedish company Svenska Resenätverket AB has a very handy website that searches for flights offered by its partner travel agencies:
or http://se.flight24.com/ if you are of Swedish persuasionhttp://flygresor.se/
More often than not, prices are cheaper than what you'd be able to find using other search tools like the ITA matrix, or what the airlines offer for the same fare at their websites.
The results are sorted by price, and you can see available flight times between destinations to adjust to your needs. As per EU consumer protection rules, the fare that you see is the fare that you pay (unless the airline charges extra for optional services like checked baggage, for example).
You can be quickly redirected to the travel agency that sells the particular ticket, and for most sites, the travel reservation will be prepared for you with your search data, waiting for you to fill in the remaining details before payment.
I travel mainly in Europe, and the results are consistently very good. I can't comment how well it works when both destinations are outside Europe, though.
Sadly, currently it's not possible to search for +/- days within the specified date.