This WordPress plugin that will allow you to display an advertisement in a modal box. You can show an image, SWF file, an iFrame or a video from YouTube.
So you’re going to manage the time during which the advertising will continue to display (the duration is automatically obtained if you use a YouTube video), how long after a new advertisement will appear (Cookie), the link clickable area (over advertising).
You can also choose the color, size (unless image because it is automatic), the opacity of the mask that appears over the page …
All these changes can made on the plugin settings page.