In fact, if you have an iPhone or iPod Touch, you’ve probably already done it on iOS 5.
Here’s how it works.
- Hold down the key for the appropriate letter (e.g. A if you want ä).
- A window of character options for that letter will pop up.
- Click the one you want, or press the corresponding number on the number keypad.
This should work with all compatible software (e.g. Word, Scrivener, TextEdit, Pages, Evernote), but it only applies to letters, not characters like © or ® or ∞.
If your OS version won’t let you access the characters as described above—or you need other types of special characters—this link has a good chart of shortcuts for special characters on Mac (and PC) keyboards.
And, of course, there’s always the Special Characters menu (Edit–>Special Characters in Scrivener or Insert–>Symbol in Word).