The SEF Test plugin allows you to turn on and off SEF from the URL for your own session, without changing the Global Configuration options. It's a very simple tool for testing SEF out on a production site with Global SEF disabled, and fetching non-SEF URLs on a SEF-enabled site. A must when building Joomla! sites.
Please note that this plugin does not work with third party SEF extensions, like sh404SEF, Artio JoomSEF or AceSEF. If you want this functionality in combination with those extensions, you need to contact the creators of that extension. This plugin is not able to add functionality that those third party solutions do not offer.