Accents, circumflexes, cedillas and umlauts. Four types of diacritic marks commonly used in European and other languages. But English rarely uses any. Often it will retain diacritics when first borrowing a word, then gradually lose them. “Depot” is rarely written anymore as depôt (or dépôt) and coöperation has become cooperation.
This lack of need for diacritics meant that in the past, when computers had more limitations in memory and processing power, the English-speaking people who developed software did not include diacritics. For people needing diacritics, this created a problem that has never been completely resolved.
For North American languages, the Language Geek provides an excellent set of fonts and keyboard layouts to assist in typing—at no charge.
Carlinga is another excellent resource. Also free of charge, Carlinga works in the background waiting for you to type a pre-programmed key sequence, then it silently jumps in and replaces the sequence with the programmed equivalent.
For example, type ,\e and Carlinga will convert it into è. Type ,/h and you get an ħ. Or ,/l to get a ł. Generally, it does not matter what software you are using, though some software programs may not support the characters (in which case you are out of luck for that software).
Another nice feature of Carlinga is that it can be modified in case your character is not pre-programmed.
Carlinga comes with a PDF file showing all the pre-programmed characters, but if you need to find a character not in the list, see List of Unicode characters. If you have Word for Windows, you can also find characters through the insert symbol feature. On Word 2007/2010, it is Insert > Symbol > More symbols. Unicode fonts with lots of characters to look for include Arial Unicode MS and Lucinda Sans Unicode.
Carlinga requires no installation or uninstallation.