Lightbox Pop 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 premium version lets you create 4 additional modes of popup viz. popup dialog box, full screen popup, quick box popup and quick bar. The premium version also allows you to define your own custom popups.
Fully Featured XYZ WP Popup Premium Plugin - Just 39 $