Blender
Image Manipulation, Media
Blender is the free open source 3D content creation suite, available for all major operating systems under the GNU General Public License.
Image Manipulation
Gimp
Image Manipulation, Media
GIMP has many capabilities. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, an image format converter, etc.
Inkscape
Image Manipulation, Media
Inkscape supports many advanced SVG features (markers, clones, alpha blending, etc.) and great care is taken in designing a streamlined interface. It is very easy to edit nodes, perform complex path operations, trace bitmaps and much more.