I was browsing about.com resourses and found an interesting article about raster editors that immitate traditional art techniques. Although some of them also offers tools for working with pre-existing images, the emphasis is on art and the creation process. Most of them have tools to mimic traditional artistic media such as oils, watercolors, acrylics, pencils, markers, crayons, chalk, pastels, and felt pens. Many of them also offer a variety of unusual, non-traditional tools.