Pop-Up E-mail subscription ( module) or appearing form?
You can try the Lightbox2 module 


  The Lightbox2 module is a simple, unobtrusive script used to overlay images on the current page. It's a snap to setup and works on most modern browsers.


or the CTools Auto-modal module


  Provides automatic modal support for any hook_menu() paths that have 'modal' => TRUE defined. This makes it easy to add optional modal support in any contrib module and helps reduce the burden on module developers to support modals.