From OutHistory

Windows

In modern Microsoft Windows operating systems, the keyboard layout US International allows one to type almost all diacritics directly: "+e gives ë, ~+o gives õ, etc.

Diacritics can be composed in most X Window System keyboard layouts.

Mac

On Apple Macintosh computers, there are keyboard shortcuts for the most common diacritics; Option-e followed by a vowel places an acute accent ( é ). Option-u followed by a vowel gives an umlaut (ü). Option-c gives a cedilla ( ç ), etc.