How to prevent open last projects when IntelliJ IDEA starts?

IntelliJ 14, 15, 16:

Settings -> Appearance & Behaviour -> System Settings -> Reopen last project on startup

Previously:

Settings -> General -> Startup/shutdown -> Reopen last project on startup.

If for any reason you need to change this setting without using the UI (for example if IDEA is hanging when it opens your project), you can find the setting in eg

/.IntelliJIdea10/config/options/ide.general.xml

The location of the file is documented in http://devnet.jetbrains.net/docs/DOC-181

The specific setting you need to change (or add) is

<application>
  <component name="GeneralSettings">
    <option name="reopenLastProject" value="false" />
  </component>
</application>

Mac

In the latest version of IntelliJ, the location of this file on a Mac is

~/Library/Preferences/IntelliJIdea2018.2/options/ide.general.xml

Settings Repository

If you have the settings repository enabled, then it can be found here:

~/.IntelliJIdea${idea_version}/config/settingsRepository/repository/ide.general.xml