This is an application to trap Alt-key presses to pop up a selector for accented keys not normally available from the keyboard. It is modelled on the tap-hold character selector built into Android. It is highly configurable.
A certain person who has written really fancy keyboard drivers says that intercepting keypresses in the way that MoreKeys does is a God-awful thing to do, a travesty, a world-shattering epic fail, yada yada yada.
Until such time as their super-sexy driver is available to the rest of us, your choice is to use MoreKeys, prat around with the !Chars application, or attempt to remember the Alt-keynum codes of the special characters you wish to use. My Pi runs MoreKeys and it sits quietly in the background until I need it.