Autodesk SketchBook used to be a paid application, but now it can be downloaded on Android and iOS absolutely for free. I like this app for its simple interface and exclusive Copic Ā® color library. The application also allows you to export your work in JPG, PNG, BMP, TIFF, and PSD formats. I am inspired by watching the paintings of my favorite artists, it gives me an unreal boost in creativity. Check these open images artworks, there are a lot of paintings collected there, starting even from the Stone Age.