Desktastic is not my favorite app from the good people over at Panic, yet it still makes for a good screen drawing app.
The program allows you to draw lines and type text onto your screen. This can be of good use for showing things to others on your screen, preparing examples or a presentation. Once installed and launched, you'll find the different tools available on the left of the screen. These had a tendency to hide behind other windows, luckily there's a button to bring them forward.
The line drawing and text tools had a good feel to them and I got around to filling up my screen with indications, lines and arrows in no time.
If you have a tablet, you'll find Desktastic great to work with though.
I was a little disappointed by the limitations in the program though. For one, the choice of colors is fairly limited. You can't really change size and there are only a few width and opacity graduations to choose from. I would also like to see a bigger choice of brushes and pens.
If you're looking for a simple screen drawing application, then Desktastic is for you.