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.

1) Corel Painter – (WIN/MAC)
Corel Painter is like a well-stocked artist’s studio, without the mess. With textured surfaces, brushes, and tools, you can mimic painting and drawing with chalk, pastels, watercolors, oils, crayons, pencil, felt pens, ink, and more. While its strong point is its artistic toolset, Painter also offers features for photo enhancement, Web graphics creation, animation, and working with text.
Price range: $248.99 – $399.35
Limitations: 30-day trial
Average expert reviews: 4.5/5

2) Microsoft Expression – Free (WIN/MAC)
Expression was the first graphics program to allow users to create brush strokes from arbitrary pictures, and remains the most artistically-oriented vector-based drawing package today.
Price: Free

3) ArtRage (WIN only)
ArtRage is a fun little free program for experimenting with digital art. Although it has some annoying bugs and limitations, it’s very user-friendly and could not be easier to learn.
Price: Free
4) Alias SketchBook Pro (Windows and Macintosh)
Alias SketchBook Pro is an innovative freehand drawing and painting program designed specifically for Tablet PC and graphics tablet users on Windows and Macintosh. Although it does not offer the same variety as other artistic software, it is extremely easy to learn and use.
Price: $179 ($79 students and educators)
Trial: 15-day trial , with some limitations on use after it’s over.

5) Deep Paint (Windows)
Deep Paint is a standalone program and Photoshop-compatible plug-in that brings real-time material-based 3D paint rendering to your digital art.

Price: $249+shipping
Trials: no trials available

6) Project Dogwaffle – Free (Windows)
Project Dogwaffle, “the unnatural paint program,” is a painting and animation program for Windows with many unique tools for creating pictures. You can create your own brushes (including animated brushes), mix colors naturally, and apply a number of special effects.
Price: $67
Trials: free light version available .

7) Deleter CGillust (WIN)
If you’re into the Anime and Manga illustration style of Japanese comics, have a look at Deleter CGillust, a computer drawing software originally developed in Japan and designed for drawing in the Manga style.
Acverage price: $60-$75
Trials: no trials found

8) Pixarra TwistedBrush (WIN)
TwistedBrush offers a collection of realistic natural brushes for media such as watercolors, oils, acrylics, pastels, oil pastels, charcoal, pens, pencils, and markers.
Trials: 15-day trial
Price: $70

9) PhotoArtMaster (WIN)
Unlike some similarly advertised products, the images you create in PhotoArtMaster really are your own unique creations—not like one-click computer-generated “art.”
Price: $495
Trials: 10-day trial

10) Studio Artist (MAC)
Users can paint and draw manually with configurable tools, or they can use automatic painting actions to intelligently paint an image with artistic effects.
Price: $379
Trials: free demo version available

Related links:
About.com full reviews