Here's one I was using for awhile. You can set it so it produces a pop-up the first time they enter your site for that browser session. I stopped using it because it made my sight jumpy.

http://javascriptkit.com/script/script2/postit.shtml

I did find another script there which is similar which uses a standard message box to display the message. I'm thinking that won't cause the jumpiness, but I do like the first script because I can specify how large the pop-up should be so I can cover must of the content.