TheThe Sliding Panels
Installing and activating TheThe Sliding Panels plugin on your WordPress site will cause activation of the following widget areas: Top Sliding Panel, Bottom Sliding Panel, Left Sliding Panel, Right Sliding Panel, and Modal Window. You can set all parameters of a panel as well as add any widget into each Sliding Panel/Modal Window widget area.
Use WP Plugins like Widget Context or Widget Logic to add logic to the widgets you are placing into the widget areas of the sliding panels.
Suppressing Cookie is an advanced option that helps you control your panel/window opening logic. It allows you to keep your Sliding Panels deactivated when some cookie is present on the visitor's computer.
For example, if you want to display your Modal Window with your Newsletter Opt-In Form only for those visitors who have not been subscribed yet, you can choose to do so. In this case, when someone subscribes to your newsletter, you can set a cookie 'Subscriber' to him. Then, enter this name in the 'Suppressing Cookie' field of the panel. And you are done!
Those visitors who have already subscribed to your newsletter will not see your modal window anymore, while new visitors will continue seeing it until they subscribe.