Speedo Popup for Wordpress
It comes with more than 14 predefined skins, 15 predefined CSS3 transitions and 7 predefined jQuery transitions ready to use in your wordpress site.
The backend gives you almost unending possibilities of making changes and customizing the popup for your needs.
You can create modal windows and use them in a page, widget, or short code. You can further customize the popup in the place you are using by setting advanced options.