Joomla! Documentation™

A common example of using plugins is to run the content plugins on some text. This is useful if you want to support plugins that usually work on Content from a custom extension. For the content prepare trigger you can simply call:

$text = JHtml::_('content.prepare', $text);

For any other content triggers you must call:

$article = new stdClass;
$article->text = $text;

// add more to parameters if needed
$params = new JObject;

JPluginHelper::importPlugin('content');
$dispatcher = JDispatcher::getInstance();
$dispatcher->trigger('onContentPrepare', array('some.context', &$article, &$params, 0));

You might want to look at core components (for example com_content) for an example. See the triggers page for the possible content plugin triggers.