LePopup WordPress Plugin by CodeCanyon
LePopup is a lightweight not just popup plugin useful for several purposes easy to manage by everyone! It features lite popup as well as initial page load plugin to act like tooltip or advertisement. If you want to show popup in sessions, there is available option to set session in hour interval. Try LePopup and you will like it!
LePopup demo: http://codecanyon.net/item/lepopup-wordpress/1129913