The phrase finder site is great! It is often a start for my research on the origins of phrases when I write that type of article, and often quoted as the source as many sources are mentioned on the pages.
I was at Liverpool Street station at the weekend, and was mildly shocked (although not, being me, actually offended) to walk past a (shut) kiosk proudly displaying a greetings card with the message "KEEP CALM AND FUCK OFF" to any and all passers by. I suspect that Mary Whitehouse would not have approved...