MakeHuman - Generate human models with skeletal animation data. Like "The Sims" character editor, but can output Wavefront OBJ. They aim for movie grade, but also have low-poly models too.
BlendSwap - Blender Scene sharing site, most restrictive license permitted is CC-BY-SA, my quick survey indicated most are CC-BY only.
Blender Materials - Materials for use in Blender, good inspiration for writing your own real-time stuff, or use them in cutscenes.
Carnegie Melon University Mocap Data - Poke around some people have cleaned the data up
Ohio State University Mocap - They just restructured their website, but the data is still there
FatBOX Software 3D models [Facebook login required] - Free models, mostly mobile phone resolution
Arbaro - Generate movie grade trees (700k polys)
ngPlant - Generate real-time grade plants/trees with real-time preview
tree[d] [win32 only]- Generate real-time Trees
cgTextures - High res textures, many are tileable
Mayang Textures - Huge amount of textures, none are tileable but easy enough to make tileable with Resynth.
Backyard Ninja Free Stuff - 2D tileset assets.. really nice pixel painted tiles for a platformer.
The FreeSound Project - Huge amount of sound effects, licensed by CC-Sampling Plus
ccMixter - CC licensed music
HasGraphics - CC licensed artwork listing, mostly 2D tiled.
Lost Garden - Webpage of the 2D guy behind Tyrian, lots of free top quality 2D artwork.
MyPaint - this one's aimed at Artists, as opposed to The Gimp which is squarely aimed at programmers.
The Gimp - The Swiss-Army-Chainsaw of free 2d image editing
Gimp-Texturizer - One click make an image tileable (didn't work for me on windows)
Resynthesizer - The original "context aware fill", although it does much more than that. Load an image, in gimp click "Filters->Map->Resynthesize" and it will make most images brilliantly tileable.
AutoStitch [win32] - Joins several photos together, fixing scale, colour and exposure to make one homogeneous image.
Paint.NET [win32] - A lightweight and fast paint app that's great for quick work.
Inkscape - Vector graphics editor
GrafX2 - PixelArt oriented program, pining for the days of the Amiga 500 and Deluxe Paint
Tiled - Map editor for making Orthogonal and Isometric tile-based maps.
The Compressionator [DirectX] - Tool from AMD for generating compressed textures and mipmaps.
SSBump Generator [win32] - Generates bump maps from height maps. Can also generate height maps from regular images.
NeoTextureEdit - a LGPL3 procedural texture editor
Blender - 3d modelling tool, great for cleaning up mocap data & making models
MeshMixer - From their site.. "meshmixer is a free tool for making crazy-ass 3D stuff without too much hassle. Or boring stuff too. You decide.". Basically drag and drop bits of things onto other things, and it makes the meshes fit. I think it will be great for baking clothes and stuff onto existing models.
Aqsis - Renderman compatible render, good for your cutscenes and as a replacement for Blenders built in renderer.
Normal Mapper [win32] - Can generate normal maps from a high poly model and a low poly model.
RenderMonkey [win32] - Tool for prototyping shaders, with real time preview.
LMMS - MIDI Tracking tool (making music), the best free one I've seen by miles. Looks like it would happily compete with the commercial options.
Open ModPlug Tracker - A Mod tracker, very primitive looking interface, but don't let that fool you, it's very capable.
Ardour [Unix, OSX] - Non-Linear-Editing for Audio. Also capable of real-time mixing. It's an industrial grade tool.
Audacity - Audio tool, great for cleaning up samples and the like.
Lightworks Public Beta [win32] - Industrial grade Non-Linear Video editing suite.
Aviary [Web-based] - Looks like it does everything in this list except 3D modelling.