I've tried a number of different pop-up extensions but they were difficult to style or didn't do everything I wanted them to do. As is so often the case, NoNumber had the answer with the Modals extension. In general, the defaults for the plug-in are sufficient for what we want to do.
The pop up is being used on a restaurant site and they want their daily specials to pop up regardless of which page the visitors accesses first and they only want the pop-up to appear once in the session.
In order to make it easier to maintain for non-technical users, I like to put the text that will be displayed by the popup in an article and then use NoNumber's Articles Anywhere to display the text of that article with Modals. In the example below, the article with ID number 7 is displayed.
Add the following code to your template's index.php file.
{modal html={article 7}{text}{/article}|open=1|width=275|height=419|openOnce=1}{/modal}
I used the following style sheet overrides in my template to eliminate the full background of the dialog and add in a simple graphic (popup_blank.png) that is in the images directory of the template.
#cboxLoadedContent {background: url("../images/popup_blank.png") no-repeat scroll 0 0; background-color: none;}
#colorbox {background: none; box-shadow: none; border: none; font-family: 'Rock Salt'; color: #D1CBA8;}
#cboxLoadedContent {margin-top: 0; top: -45px; position: relative; }
#cboxTitle {border: 0; position: relative; }
#cboxClose {z-index: 9999; right: 55px;}
#cboxLoadedContent p {padding-right: 20px;}
Go to the Plugin Manager and locate System - NoNumber Modals.