German: ss instead of ß, ue instead of ü, ae instead ä etc.
Italian: e' instead of è, or perhaps é, I'm not sure
Esperanto: cx instead of ĉ, ux instead of ŭ etc.
However, in French there aren't any such conventions (annoyingly enough!) except perhaps for oe instead of œ, which is fairly trivial.
I have two questions: (1) Does anyone have examples from other languages? and (2) Does anyone know how such conventions developed?
(I should probably point out straight away that the 'x' thing in Esperanto is NOT what Zamenhof himself proposed; apparently he proposed using 'ch', 'sh' etc. So it must have developed in some kind of 'natural' way, like in any other language.)