It's called the federalist flag, it was used as the "european flag" before this one.....
Interesting, thanks for that. I see now that it is a green E, but this flag was never officially adopted by the EU (the ring of stars flag was adopted in 1995). Using the green E flag is quite a political statement - see http://en.wikipedia.org/wiki/Federalist_flag
The other flags which are visible are Italy, Ireland, Denmark, Greece, Belgium, Germany, United Kingdom, France and Luxembourg. These are all EU countries of course, but I have no idea why the puzzle was produced.