If you want to write and paint on your Mac's screen without doing any permanent damage, Deskscribble for Mac OS X (10.6.7 or later) is for you. It not only allows you to draw on top of your desktop and atop all open windows, it's also an easy-to-use whiteboard, blackboard and you can even use it for your own and your kids’ scribbles.
Deskscribble -- available in the Mac App Store for US$9.99 -- isn't something I will personally use very often. However, if I were still a Mac-using teacher (as I once was), it could be invaluable. Deskscribble lets you work with either a whiteboard or blackboard interface. It supports written text, diagrams and more, so teachers and others making presentations can "mark" on their desktops -- circling key concepts, underlining important words, etc. .-- and share this with others using a projector.
Besides teachers and those doing business presentations, Deskscribble is useful for those demoing software products and instructing others on how to use apps. You can use your currently active desktop background as an image for annotation.
You can also import images from apps such as iPhoto. However, export options are limited to saving your work as a PNG file.
Deskscribble offers a wide variety of colors, variable brush widths, decent pressure sensitivity and multiple un-dos. If you're a graphic designer, you'll also appreciate the Wacom tablet compatibility.
Deskscribble will autosave your drawings, which is rather convenient. When you bring up Deskscribble after quitting it the last time all your drawings are there where you left them.
There are some drawbacks, however. Deskscribble's interface overall is pretty good, but I had trouble finding the "soft eraser" function. You can't move the interface palette. And hotkey activation and keyboard shortcuts would be handy.