IPEVO AnnotateIPEVO Annotate. This is a free download from IPEVO. It allows you to mark up anything that displays on your computer. I use a Wacom tablet to annotate but this will work with a trackpad or a mouse. You will see I am using a mouse in the demo. This is a great way to screen record and annotate on PowerPoint, Keynote, or Google Slides. You can also pop up a whiteboard to solve problems.

Note you cannot annotate in a full-screen presentation. You need to present in the window. This took me a while to figure out. Here are the directions for doing this in Keynote, PowerPoint and Google Slides. 

 

You can also use IPEVO Annotator in Whiteboard mode. Here is a video to explain this.

 

For an ipad like experience, you can use a Wacom graphics tablet that comes with a pen. This will allow you to annotate with the pen and is much easier to draw equations with.