The easiest and probably best way to type in special characters such as accented letters or mathematical symbols is to use the compose key.

It's not turned on by default, but it's easy to set up.

1. In your GNOME desktop, go to System->Preferences->Keyboard
2. Click on the Layouts tab
3. Click on Other Options...
4. Find Compose key position and click on the triangle
   to the left to open it up
5. Tick the Caps Lock is Compose box

Now you can type lots of useful things.

For example, Caps Lock, then ', then o, will turn into ó.

Other examples. Press Caps Lock, then the first key, then the second key, and it turns into the Result.

First key	Second key	Result
' (single quote)	o	ó
" (double quote)	o	ö
` (under the ~)	o	ò
s	s	ß
,	c	ç
<	<	«
o	c	©
=	e	€
x	x	×
:	-	÷

You can change the o to any other vowel to get the same accent on that letter.

For a full list, see the Wikipedia article on the Compose key.

For Windows, there's a program called AllChars that lets you do the same thing with Control, or any key of your choosing.