What is the mu-plugins folder?
Users of WordPressMU have long been aware of a folder within wp-content called mu-plugins. Obvious conclusions were drawn because of the name and where it was, but this actually stands for “must-use” plugins. Why? Because any code (plugin) placed in the folder is run automagically, as if it were part of WordPress itself. (Yeah, that label was news to me too.)
Support for this folder has actually been in single WordPress since 2.8.
In WPMU, you never saw any reference to this folder from the backend at all. Just drop the code in, and the plugin was already running.
In 3.0, support for this is naturally still there, but the folder isn’t created for you. If you do encounter a plugin (like domain mapping) that needs to run in this folder, simply create it via ftp or through your webhost’s control panel.
A listing! You’ll note on this example site (other than we really need to clean out the plugins we were testing), must-use plugins are now listed. This will only be visible to Super Admins – regular administrators, even with access to the Plugins menu, will not see this.
While you may be tempted to toss all your plugins in there – don’t. Like stated above, the code placed in here runs automatically and (depending on how the plugin is coded) on every single page load. Also, any files in folders will not be seen. There must be a physical php file there to be read.
Popular plugins that do belong in this folder are ones like Domain Mapping and New Blog Defaults. Please make sure to always read the readme.txt included with any plugin, as it will specifically state if it should be installed in this folder.