This option should contain a comma separated list of plugins. Plugins are loaded from the "tinymce/jscripts/tiny_mce/plugins" directory and the plugin name matches the name of the directory. TinyMCE is shipped with some core plugins these are described in greater detail in the Plugins reference.
TinyMCE also supports the ability to have plugins added from a external resource, these plugins needs to be self registrering and loaded after the tinyMCE.init call. You should also prefix these plugins with a "-" character so that TinyMCE doesn't try to load it from the TinyMCE plugins directory.
There are many third party plugins for TinyMCE some of these may be found under "Plugins" at SourceForge and if you have developed one of your own please contribute it to this project by uploading it to SourceForge.
Example of usage of the plugins option:
tinyMCE.init({
...
plugins : "table,contextmenu,paste,-externalplugin"
});