I normally use Fedora for my day to day work, and I wanted to be able to type accented characters, specifically French accent marks like in the words "très" and "ça va". It took me a good bit of searching before I found an easy way to do this so I thought I could share the information here.
I found this page on Accented_Characters, but the "Alt Gr" didn't seem to work for me. Eventually I found a hint on the ubuntu forums and was able to find the Fedora equivalent.
So here is the solution (using Fedora 7):
Go to System -> Preferences -> Hardware -> Keyboard, select the Layout Options tab, expand "Compose key position" and choose your preference (I chose the right Alt-key). This seems to make the compose key into the "Alt Gr" key on the other page I found.
So now things like (Alt + `, e) will give me è. Have a look at the wikipedia entry for additional information on using the compose key.