http://www.murky.org/cryptography/classical.shtml
The section on "classical cryptogrpahy" is interesting. In particular the "Caesar shift" and "monoalphabetic substitution" are nice.
They are nice because they are a) simple and b) the sort of thing that was used in history and so could equally be used in a fantasy setting.
They are also crackable, because after all, we want to leave some clues or chance don't we
