Rediscovered in a secondhand bookshop in 2000, artistic works created by the Crown have a copyright that expires after 50 years so it was in the public domain. There have been attempts to copyright it though. You have probably seen it everywhere in recent years. It's been so big that it's been plastered all over all sorts of products and even on office walls etc. The argument is that the words within it are far too commonplace to copyright or trademark.
There have been many many parodies on it. Some of them are even getting a bit grating and don't even make sense from a 'this happens to be insultingly terrible grammar here' standpoint. hehe.
But there are also quite a lot of fun and pretty cool ones. My own personal favourite applications of it: a t shirt i bought in the UK last year said 'Keep calm and kill zombies' which I wore to last years annual Zombie Walk, my keep calm and bake on t towel and the keep calm and sing soft kitty necklace from Etsy.
It might be getting a bit overdone but personally I do love the origins - it's very REAL and I love the variety of idea that other Etsians are coming up with:
Soft Kitty Tile Pendant...
Keep Calm and Carry On ...
Keep Calm and Drink Win...
Keep Calm (Yoda): a pen...
Keep Calm and Soak On V...
Keep Calm and Craft On ...
Zombie Shirt - Keep Cal...
Don't Keep Calm, Fu...