How does a plugin include it's own config options?
You can add a config.php file in the root of your plugin's folder (craft/plugins/pluginhandle/config.php) that specifies the default values of your config items by returning an array of key value pairs for config settings and values.

If a user wants to change any of those settings, they can add a pluginhandle.php config file to craft/config/pluginhandle.php that returns an array with the overridden key value pairs.

From your plugin, if you call:

craft()-&gt;config-&gt;get('key', 'pluginhandle')

then Craft will return the user supplied value, if it exists, and fallback on the plugin supplied default value, if it does not.

Also worth nothing that all of Craft's multi-environment config stuff will work in this context for the plugin, too.