XYZ WP Popup is a handy wordpress plugin which allows you to create a non-annoying popup with custom content in your site. You can configure popup position settings (height, width, top, left), display logic settings (time delay after page load, number of pages to browse, repeat interval) and style settings(z-index, overlay opacity, color, border etc).
The plugin lets you create 5 different kinds of popups viz. lightbox, dialog box aka message box, quick popup box, full screen popup and quick bar.