This WP Pre Post Advertising 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.