Exit pop up with registration form
If you're looking for a module, try Splashify. Splashify is although not available for Drupal 6. Tinybox can help too but I haven't tried it myself.

You can also create this yourself, simplest way would be to change your front-page in configuration in toolbar, create that page, in drupal template, write something like this:

&lt;?php
if (drupal_is_front_page()) {
    ?&gt;
/*Your Splash Screen Code*/
&lt;?php
    }
?&gt;


Your Splash Screen Code can be a simple HTML Model, see this Simple Tutorial.