Add a modal block that opens at load the page

I think there should be a way to make something like the modal block (either a new block or an enhancement to the existing one), but instead to load after clicking a link, it should be opened by default when one enters to the site, either as a generic popup or as an sticky bar at the beggining or end of the page (this is intented to be used more on the site-wide template than specific entries). The «closing» of it can have the option to be cached so it don't appear to the user when one re-enter to the site.