We get a fair number of Presenter '09 users who ask about how to create a pop-up box that learners can invoke by clicking on an item that appears on the course slide. It's actually pretty easy to do — it just involves using some simple PowerPoint hyperlinks and creating a separate slide that acts as a layer over the original slide.
The effect works great for situations when you want to provide learners with optional context information, or if you want to create a click-to-reveal interaction. It's a nice way to give learners the feeling that they're exploring your content by "uncovering" items or information on your slide.
Here's how to build the effect: