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:
$dispatcher = JDispatcher::getInstance(); $item->text = your_text_area_item; $item->params = clone($params); JPluginHelper::importPlugin('content'); $dispatcher->trigger('onPrepareContent', array (& $item, & $item->params, 0));
Note in Joomla 3.x JDispatcher is deprecated in favour of JEventDispatcher. However as JEventDispatcher doesn't exist in Joomla 2.5 - this code is needed for your extension to support both versions of Joomla.
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.
Also for PHP5.3 compliance please look at the discussion page.