Inkscape – Vector Illustrator’s Toolbox
Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, CorelDraw, or Xara X, using the W3C standard Scalable Vector Graphics (SVG) file format. It has some very cool tools, and can be very effective in graphic design work.

Blender – 3D Modeling, Rendering, Animation, Video
Blender is the free open source 3D content creation suite, available for all major operating systems under the GNU General Public License. Its interface is tough to learn at first. But, after some hours with video tutorial instruction or books it becomes a very valuable asset to your personal production pipeline.

GIMP – The Photo Shop Of The Open Source World
GIMP is the leading photo editor in the open source world. It works like other (proprietary) raster editors including Adobe Photoshop, Jasc Paintshop Pro and Microsoft Paint. You can learn more about the software and even download a copy at http://gimp.org.