My client wanted not only a popup window, but a sliding popup window. The kind that slides across the page when you first get to a site. Or maybe it slides in after you've been on the site for a while.
I searched high and low for a jQuery popup that would do just that. It wasn't a matter of not being able to find jQuery popup plugins, it was a matter of finding one that would slide and one that I could get to work in WordPress. I'm sure a lot of that problem is my lack of understanding as to how jQuery works in some cases, but I needed one fairly quickly that was easy to use and install.
I finally landed on bPopup. It does exactly what I want to do. It lets me choose whether I want it to slide in and out or up and down. It was quick and easy to install and he even has a jsfiddle that let's me play around with the code before I install.