Every Snippet can have tags and a format (which are custom taxonomies) attached to it. On display, the plugin will search for snippets with the same tag and/or format, and display one of them acccordingly. If multiple snippets are available, one of them will be randomly choosed on every page refresh.
You can use this plugin for your ads, but also for any HTML/Javascript/CSS code you wish to display on multiple pages.
This plugin is available for download to be used on your WordPress self-hosted installation.