Paper by Fifty Three (http://madewithpaper.fiftythree.com/ (http://madewithpaper.fiftythree.com/))
The app is free...however, you get only 1 tool (the charcoal pencil, I think) - & can purchase the marker, pencil, ink pen, paintbrush, & paint mixer - all in all I spent $8 bucks.
Here are a few samples of what I like to do to relieve a little stress...I rough out the work w/charcoal, then gradually fill in details & color.
(http://i226.photobucket.com/albums/dd27/gablabs/Fifty%20Three/4x4_Paintings_zpsfd603dda.jpg) (http://s226.photobucket.com/user/gablabs/media/Fifty%20Three/4x4_Paintings_zpsfd603dda.jpg.html)
(Cave Painting, Independence, Jazz Band, Spanish Street Scene)