Blender has that grease pencil tool if thats any good to you... Let's you draw in the viewport and even animate it I think. Only really useful if youre animating in Blender too I guess...
Ah cheers Jermaine, Storyboard Pro looks a little more than I need, and a LOT more than I wanted to spend too but I found Pencil Check Pro on their website which looks absolutely perfect and it's like $100.