Most plugins that are installed into /wp-content/plugins need to be activated. However, it seems that if you drop scripts directly into /wp-content (object-cache.php comes to mind) they are activated automatically.
How is this?
How do plugins get executed automatically from wp-content?
I know, myself, using a multi site WordPress install, plugins dropped into the mu-plugins folder (wp-content/mu-plugins/) are activated automatically. In this case mu representing must use.
However, I am not 100% positive if this is dependent upon the type of install a single site blog install or multi site/network install.
I found an article outlining most of the information about mu and drop in plugins that I know here.
Additionally, if you are looking specifically for how these plugins are loaded I opened up /wp-admin/includes/plugin.php and found get_mu_plugins() on line 279.
If WP_CACHE is set it expects in the wp-content folder the cache php file.
The way to disable the WordPress cache is to define another cache, also in the wp-content folder, called object-cache.php.
found google at the top: http://funcdoc.wordpress.com/2008/01/30/replacing-wordpress-object-caching/
So answer: no (almost) no plugins get executed from wp-content. to execute them automatically see the other answer on the drop ins.
There is number of specially named files that get loaded automatically when present. They are called
Drop-insand are displayed in Plugins interface when present (since some recent WP versions).They can be added by regular plugin or manually.
Note that this only works for specific files. Those that are not recognized as drop-in won’t run automatically and will require activation as usual or placing them in
mu-plugins.