If you press and hold a letter in OS X, a pop-up will appear with the various accented versions of that letter. If you don’t use this and it bothers you, you can disable it.
defaults write -g ApplePressAndHoldEnabled -bool false
Restart your Mac or logout/in to make the changes appear
Revert back to default with:
defaults write -g ApplePressAndHoldEnabled -bool true
Screencast: