Reset IntelliJ UI to Default

On Mac OS for IntelliJ v12, shut down the IDE, and then you can execute:

rm -rf ~/Library/Preferences/IdeaIC12/*

Restart the IDE, or open a pom.xml of your choosing. You will be asked whether you want to import the preferences from an existing IntelliJ instance. Select the "No, I do not have a previous IntelliJ version" radio button.

Recent Versions

Window -> Restore Default Layout

(Thanks to Seven4X's answer)

Older Versions

You can simply delete the whole configuration folder ${user.home}/.IntelliJIdea60/config while IntelliJ IDEA is not running. Next time it restarts, everything is restored from the default settings.

It depends on the OS:

https://intellij-support.jetbrains.com/entries/23358108