I finally found the plugin I was looking for. There’s all kinds of popup makers for regular web sites, but to do it for WordPress is a bit tricky, for me anyway. The site I found is MaxBlogPress and they specialize in really useful and free WordPress plugins. This Unblockable Popup Plugin is highly configurable. For example, you can tell it when and how much per visitor you want it to show. I’m still putting it through it’s paces, so I haven’t learned everything. This is just what I wanted to put opt-in box popups on a few of my niche blogs. Check out the free Unblockable Popup WordPress Plugin.