WordPress has quite a few hidden features.  My favourite so far: MU Plugins, or “must-use plugins”.  Introduced with WP 3.0, the code says:

/**
 * Check the mu-plugins directory and retrieve all mu-plugin files with any plugin data.
 *
 * WordPress only includes mu-plugin files in the base mu-plugins directory (wp-content/mu-plugins).
 *
 * @since 3.0.0
 * @return array Key is the mu-plugin file path and the value is an array of the mu-plugin data.
 */

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