Simple diagrams (link) is a nice little tool to create simple sketches in the spirit of Dan Roam's book "The back of the napkin" (review). You can either use it as a sketch tool to develop ideas, or as slides in your presentation. The extreme scenario would be to create an entire presentation out of these types of diagrams.

The program uses aggressive pop up messages to get you to use the full version. There are more subtle ways that will get to the same effect.