Well, you could add a questionnaire to the main page of your site. People would have to at least login as a guest though (use the 'login as guest' button).
You can directly access the surveys in phpESP through the link,
Replace the [surveyname] with the name you gave the survey. But this won't use Moodle users, and isn't working within Moodle. You'll also have to add the line "require_once("../../../../config.php");" to the top if your /mod/questionnaire/phpESP/public/survey.php file (or use the one I've attached here - I'll move it into CVS).
But if you really want to do this outside of Moodle, I'd suggest just using the phpESP application itself. You can get it at: http://phpesp.sourceforge.net/.