One easy way to send your visitors a welcome message, notice, or advertisement is to add a popup to your site. The WordPress repository has a new plugin called Cool Fade Popup that allows you to create unblockable popup ads for your site.
Try out a demo on the plugin’s homepage. It’s easy to install and configure.
If you’re looking for a more full-featured option, check out the WordPress Popup Plugin. It is compatible with WordPress Multisite and allows you to display a fancy popup to visitors sitewide or per blog. It can also recognize if your visitors have been there before, how many times they have seen the popup, if they are coming from search engines, etc and you can modify the behavior of the popup as you wish.