Look on JED there are plenty of "splash" page extensions. However, I have to say that generally splash pages have fallen out of favour and are generally frowned upon nowadays. You can also do it with just joomla itself by create a menu and menu item within it that just shows one module and making that the default menu. Then have you content of that menu item link to another "page" where an alternate "normal" menu is shown showing the normal page content.
Regards, Mark.
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