FF

or hexadecimal (0xFF) for 255,decimal

The old French franc was usually written FF by the outside world (more so than that that ISO standard FRF).

Also the Fantastic Four in Marvel Comics.

The most interesting thing about ff is that its having been a ligature in printing has been enshrined in some surnames. Think of an old Gothic capital F: it's an ornate confection of pen-lines and swashes, and it actually looks hollow inside. What this means is that it looked quite like two lower-case ff joined together. So you got surnames like ffrench and ffolkes and ffrangcon. Sadly, in these computerized days, these old names seem to be dropping away. The actress Gwen Ffrangcon-Davies seems always to have been written with a capital F.

Those that do remain as proud bastions of ff-ness seem drawn to the eccentric, to hyphenation, and to nominative determinism. The most famous bearer of an ff-name is probably the South African theologian and anti-apartheid hero, The Very Reverend Gonville Aubie ffrench-Beytagh (1912-1991), the Dean of Johannesburg. (I believe the second element is pronounced "beater".) There is also the cartoonist ffoulkes from Punch with his characteristically untidy style.

Trawling the Web, I found a Professor ffrench of the Department of French, with his turgid post-structuralist analysis of Bataille's L'Histoire de l'oeil, and two scientists both called Professor ffrench-Constant. It would no doubt be better if they were physicists and thus had more hope of getting a constant named after either of them, but they're biologists both, and have compensated by choosing interesting department names: Richard ffrench-Constant is Professor of Molecular Insect Science at Bath, and Professor Charles ffrench-Constant is at the Cambridge Centre for Brain Repair.

Charles explains his work at www2.bioch.ox.ac.uk/~oubs/past/ffrench.html
Richard wins a prize at www.bath.ac.uk/pr/releases/rswolfson.htm