What font does the poster use?
Anyone under the age of 20 or so might not believe this, but there was a time when computers didn't exist. The Keep Calm poster was hand-designed by a (sadly) anonymous civil servant and as such doesn't use a standard font. However a very clever chap called Keith Bates has painstakingly recreated the original font as closely as possible by taking examples from the original Keep Calm poster and the two other posters in the same series. You can buy his font from
his website.Also, according to discussions at typophile.com the closest pre-existing match is a font called Avenir by the famous font artist Adrian Frutiger, which can be purchased from Linotype. The next best match is a font called Gotham which is available only from its creators Hoefler & Frere-Jones.
We have both Keith Bates' 'Keep Calm' font and the Avenir font available for use on the Keep Calm-o-matic, allowing for even closer reproductions of the original poster.