There are not many drawing applications which are cross platform but Sketsa SVG editor uses SVG as its native file format.

What is SVG you ask? SVG is basically an open standard language for drawing two-dimensional (2D) graphics in XML developed by W3C. SVG therefore benefits from the various XML tools that make it so easy to manipulate, generate, search and edit SVG files. The developers also guarantee that as a result, Sketsa also produces very clean SVG XML code. The editor is basic but has everything you need to produce some impressive results. It features various tools for optimizing content creation and gives you a decent feeling of control over what you are creating. The design tools allow you to select, draw, and edit objects easily. It supports SVG specific shape tools and supports additional illustration tools such as the Pencil tool for free-form shapes and Pen tools for curve shape. You can also transform objects or drawings by rotating, skewing and scaling.

Sketsa SVG editor is very basic but for a free application, it packs a powerful punch as an artistic tool and is perhaps best used as something to train on before moving onto something bigger.