Inkflow is an iPad app for visual thinkers who like to sketch to process what they're hearing, seeing, and learning. Inkflow can be used to sketch mind maps, draw pictures, write free hand, and just about anything else you might do with a pencil and paper. You can organize your Inkflow pages into books.
There is a free version and a paid version of Inkflow.
The free version allows you to use black ink only and your books are
limited to twenty pages. But you can have as many books as you want in
the free version. The paid version gives you more drawing tools and
colors as well as more pages per book. (From Free Technology for Teachers)