Artweaver allows you to paint creatively with the help of a huge range of painting tools. You can create sketches from photos or just experiment with colors.
For professional users, Artweaver offers layers, group layers and support for the PSD file format. It is also possible to load and run Photoshop compatible filters (only Artweaver Plus).
Get the free version here: http://www.artweaver.de/download-en/