Here is the best drawing app for artists hands down. It just had a great update and my wife was telling me also that it is now free. It used to cost money and it was worth the price then. Now being free, everyone can benefit from it.
The intuitive design of the gestures to navigate the app is amazing. Like undo is a two finger circle rotation that makes me feel like I'm rewinding a tape. Pinching to close the book and also multi-touch functions so I can rip out a page in one book, close the book, and add it into another book without taking my fingers off the screen.
The different tools that it comes with makes for a great art app. The pen is my favorite. It makes anything I draw look amazing.
If you notice in this picture you will see some of the new tools in the update. These are very handy indeed. Check it out! Only on the iPad. It really wouldn't be useful on the phone anyways. :)