Open or Free tools for the digital artist.

Here are, in my opinion, the best free graphics applications available for Windows and a few for OS X. These tools effectively replace the need for expensive tools, like Adobe products. I use these along side some commercial applications and they integrate very well with little effort.

There are a lot of lists that consist of free Windows graphics software out here, but I actually use these products unlike many of the people listing tons of programs, and these are my personal favorites for there stability, power, and of course their licenses (open or free). If you're also wanting some nice plug-ins for Photoshop, there are some really nice ones here. I will post a "how to" to get Photoshop plug-ins working in Gimp on Linux and Windows shortly.

Windows...

MyPaint (Similar to Corel Painter) Natural painting application. Also see Pixia
Paint.Net (Similar to Adobe Photoshop) Image editor. Also see The Gimp
IcoFX Icon/Pixel editor
InkScape (Similar to Adobe Illustrator) Vector editor
Pencil 2-D Vector and bitmap cartooning and animation. Also see Synfig
Scribus (Similar to Adobe Pagemaker) Desktop Publishing.
Blender3D (Similar to Lightwave 3-D) 3-D modeling, rendering, and animation. Also see 3DCanvas
FontForge (Similar to Fontographer) Create true type fonts. Also see Win XP built in editor, type eudcedit in a run dialog.

Fyre Computational artwork based on histograms of iterated chaotic functions. Uh, yea, it makes cool images...
Fractal Explorer 2D, 3D, and Land fractals.

For a complete all in one graphics suite check out FreeSerif. Not my personal favorite, but you might like them and they integrate with each other well.

Mac OS X...

ArtRage2 Natural painting application. Free version and commercial version available. Also runs on Windows.
LiveQuartz Image editor. Also see Seashore
Pixen Icon/Pixel editor

Xaos Fractals