This Wordpress Plugin allows you to show an advertisement before showing the actual post/ page. The plugin first loads an advertisement and shows a countdown. As soon as the countdown finishes the actual post gets loaded.
You can add any type of advertisement ( Image, Flash, Video, Text, shortcode, ... ) as the advertisements are just normal posts. You can even include a full post as an advertisement.
The Plugin Admin page allows you to set the default values for the advertisements.