As stated in the comments, there is now way to call a component without proper Joomla CMS initilization.
However, you can call an Url to generate the component alone, adding these parameters "tmpl=component".
For example: http://www.joomla.org/index.php?tmpl=component
Furthermore, as @bakual notes:
To be precise, &tmpl=component will load the component.php within your active template directory, with a fallback to /templates/system/component.php. Usually that one only loads the component (plus system messages) however it could do anything you want. You could also use other values like &tmpl=foo which would load /templates/yourtemplate/foo.php