Which is correct?
Answer: Either, depending on location.
In the US, font is the standard spelling; British style was originally fount, although both spellings are used now. In Canada, we’ve adopted US style for this one, although we still maintain the British u in words like colour, flavour and favour.
Oxford Dictionaries Online provides a handy reference for British and American spelling.
See also: Font of wisdom or fount of wisdom?
Or, just for fun, go read Steve Martin’s NY Times “Shouts and Murmurs” piece: Times Roman Font Announces Shortage of Periods
Follow me on Twitter @tao_of_grammar