ArtRage is an awesome application.  It’s one of the best painting and sketching tools I know of, and you can create some amazing works with it.

“With ArtRage you can paint with oils and watercolors, sketch with pencils, spray stickers over your canvas, and much more. Stencils and rulers let you create precise shapes or smooth curves freehand. Tracing and Reference images let you load photos to recreate either by eye or by letting ArtRage select colors for you as you paint. For professional users, ArtRage offers Layers and Layer Groups, Layer Blend Modes compatible with the PSD file format, plugin Filter suport and more.”

http://www.artrage.com/

An example of an image created with ArtRage.