Blender - 3D for the rest of us.

Perhaps the most misunderstood open source package, Blender is a shining example of what happens when artists decide to develop something, and have a vested interest in getting it right. It comes complete with UV editing, rigging, animation,composition, rendering, and more features than could be properly listed here.

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Some tutorial links:
@ibiblio: [link]
@blendernation: [link]
@wikibooks: [link]

The GIMP - Image editing on a budget.

The ultimate misnomer; The GIMP is the perfect compliment to Blender. It lacks some of Photoshop's features, but these can be added with plugins.

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Some tutorial links:
@gimp-tutorials: [link]
@gimp.org: [link]

There are too many good sites to list - your best bet is to google for them.

Inkscape

Need a good, multi-platform vector editor?

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Some tutorial links:
@Inkscape: [link]

Yafray

Blender's built-in renderer not good enough? No problem! Yafray is an excellent alternative to more expensive renderers. Support for this is built in to Blender, but you can use the more up to date external version.

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Python

A free, powerful, open source scripting language. Blender has a limited built-in version, but you'll need to install this if you want to get the most out of Blender.

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