Free Software for Drawing Overlays
iMovie comes free with a Mac, and it allows "Picture-In-Picture" images. You can create art with a transparent background, using a free program such as GIMP, then save it in PNG format (not JPG) and then drop it over your video in iMovie.

For emphasizing sections of your video, you can also create a whole image that is partially transparent (like a black image that is 30% transparent) and "remove" a section of it (so the transparency of the "hole" is 100%).  It will look like smoky glass with one perfectly clear part.  When you lay that over your video, it will look like the rest of the screen was darkened except for the part in the perfectly-clear "window". Kind of a nice effect. :)