The gorgeous and oft-parodied "Keep Calm and Carry On" poster was commissioned by the British Government during WWII but was never actually used, as it was only produced for use in a last case scenario should those fiendish Nazis manage to invade. It was rediscovered in a second hand bookshop in 2000 and has since taken on a life of its own, as the image is now public domain. I didn't realise this until today, when I went Googling it.
(As a total aside, does everyone else hate this new Google Instant thing as much as I do? Charlie Brooker hit the nail on head as per usual: "it's aggressive – like trying to order from a waiter who keeps finishing your sentences while ramming spoonfuls of what he thinks you want directly into your mouth." Seriously Charlie, ditch that Konnie Huq hottie and let's do this already.)
So, to the point of this post. There's some really good versions of this poster out there (I have to say, I'm loving the nerdtastic Wrath of Khan one above) but I think my favourite one thus far is by Osmand Nosse of the very funny A Cat Is A Cat.
They're for sale in his shiny new shop for only twenty of your Earth euros, and I suggest you have a looksee. I like it a lot.