I love Skitch.
I use it in class all the time. It really helps my teaching to flow. It is a simple drawing program with all the features that you need and none that you do not.
I use it mainly to present information in a flexible and interactive way. It would be a ton less valuable if I did not have my Bluetooth tablet. Since I do I can use the tablet to write on any thing I clip from the web.
I will admit that presentations are not perfect in Skitch. However, its always available nature make is a perfect companion to the teacher wandering around the room and writing on the projector.