Cost: Free/Open Source
Website: http://www.gimp.org

The Gimp is an excellent alternative to Adobe Photoshop. It has many of the same features and is completely free and open source!

Cost: Free/Open Source
Website: http://inkscape.org

Inkscape is an open source vector graphics application similar to Adobe Illustrator. It features a “3D Boxes” tool that is truly unique, which allows you to draw a series of cubes in perspective. You may then adjust the perspective on the fly and it will apply to your entire layout!

Cost: Free/Open Source
Website: al.chemy.org/

Alchemy is a truly unique piece of software geared towards generating quick sketches, concepts, layouts and ideas. You can easily switch everything from Alchemy into Illustrator, Photoshop or any graphics program of your choice. Give it a shot, it is free after all!

Cost: Free/Open Source
Website: http://www.blender.org/

Blender is a free 3D modeling, animation and rendering program with many of the same features as high-end application such as Cinema 4D, 3D Studio Max and Maya.