These posters are so popular and everywhere.
According to Wikipedia, after 50 years of British Crown copyright issues, the design is now finally in the public domain. The Economist says the design “taps directly into the country’s mythic image of itself: unshowily brave and just a little stiff, brewing tea as the bombs fell”, hence it became popular in Britain throughout the 2000s.
This design has now been spread across the world and is very desirable mainly because of its simplicity and ability to generalize and work with most human experiences and activities. It became a way for people to share their personal experiences and thoughts without having to do too much work. (people spend large amount of time organizing these posters: http://keepcalmand.tumblr.com/)
