This is a complement to Corinne McKay's latest blog post "Inserting accented characters in OpenOffice.org". Since Corinne focuses on Linux users I thought I'd rather add Mac specific information here.
Mac OSX comes with system wide shortcuts that work in any version of the OS. And if I remember well, the shortcuts were here well before OSX.
It is possible to use them in any text editing location (a web browser field, your email, your favorite office suite, even the terminal).
To check how to enter a character that is suspiciously missing from your keyboard, you need to display the Keyboard Viewer utility, a small window that will show you where is the key that will input that character on the keyboard you are using (if you have a laptop with an external keyboard like me, the viewer will adapt its display to correspond to the keyboard you are actually typing with, as you type).
If you are used to run a strictly monolingual system (surprising for a translator), you may need to go to System Preferences -> International -> Input Menu and to check the little Show input menu in the menu bar box at the bottom of the window. Also, make sure that the Keyboard Viewer box is checked in the above list of displayable items.
Once that is done, you should have a menu bar item that displays a flag that should correspond to your default keyboard setting.
Ok. So, where are the shortcuts now ?
For your information, I use the US standard keyboard, other keyboard layouts will allow for other shortcuts (see Paul's comment on the US Extended keyboard's available characters).
Go to the flag menu, select Show Keyboard Viewer and you'll see a copy of the keyboard you are typing with displayed on your main screen.
Now, type a few lines with your keyboard and you'll see the viewer react to each key you've used: it will display each key as depressed as you type it.
Now, hold the command key and check the viewer: you'll see the viewer's command key(s) depressed but nothing more.
Do that with the alt/option key. What you see should be a little more interesting. You should have a few keys that totally change of appearance. On my keyboard, q turns into œ, for example.
Which means that if you want to type œ in your text, just use alt+q. Œ would then be shift+alt+q.
That is for direct input. What about accents ?
If you press alt you will probably see a few key that turn orange. Like alt+e. They turn orange and display an accent. alt+e displays the acute accent.
If you type alt+e, OSX will insert an acute accent at the location of the cursor but the accent will be displayed with a brownish background: OSX is waiting for you to type the letter that will go under the accent. If you type e then you'll have a nice é at the location of the cursor.
On my keyboard, the possible combinations are:
You can also have direct input for pretty much all the characters in use in European languages thanks to this system. If you need more, just go back to International -> Input Menu, add the language you need (there is a huge lot of languages available directly on the Mac) and select it from the flag menu when you want to use it.
With the US standard keyboard, I use shift+alt+2 a lot for €, alt+c (and shift+alt+c) for ç (and Ç) as well as a few others.
If you need something very exotic and you don't want to change of input system/keyboard layout (the flag menu thing), you can also access the Character Palette (don't forget to check its box in Input Menu above).
Here, the system provides you with a way to access all the characters available from all the fonts present on your system. Select an appropriate drop down menu from the top left, then a category from the section below, browse the space on the right side of the window. You can search a character by name in the search field at the bottom right. Enter note and look at the possibility offered by your fonts.
The action drop down menu at the bottom left allows you to search for the character that is selected in the front application.
Of course, the whole input system is available system-wide and works in pretty much all the applications that run on OSX.
Caveat: applications that depend on the X11 windowing environment may not respect the OSX conventions and will sometimes not accept such input.