Links submitted by Domino Marama

Lots of good quality images, many that are already setup for use as textures.

Good selection of reference images across a wide range of subjects.

Lots of landscape reference pictures and a variety of other photos.

Massive collection of content, but it's not all suitable for commercial use. Remember to check the individual licenses if you see something you like.

Free stock photo site with a wide range of content

Probably the best source for free vector clipart, though the quality of drawings varies widely.

Good selection of ready to use textures. Also has links to tutorials for the major 3D packages and a 3D gallery.

Small selection of high quality textures just for car makers.

If you want to model a real life item, you'll need the blueprints. Lots of reference pictures for modeling use.

Good selection of textures and images including close ups of things like fabric seams and other hard to find items.

Large selection of field recordings, great for ambient effects

The Freesound Project is a collaborative database of Creative Commons licensed sounds

The Blender Installation Guide from Machinimatrix covers installing and configuring Blender and the Primstar scripts on Windows.

Creating sculpted Prims with Blender from Machinimatrix shows how to create a workman's helmet.

Creating Surface textures for sculpties with blender from Machinimatrix continues the workman's helmet by explaining how to create a texture for it.

Texturizing sculpties with multiple images from Machinimatrix continues the workman's helmet with an explanation of using multiple textures to add a logo.

Many stock photos across a range of subjects. Pictures tend to be more artistic than reference, but I usually look here first as the quality is usually high.

Lots of tutorials from installation to advanced techniques such as faking fur textures with baking particles.

A short tutorial on using nodes to enable specular shading to be baked to your texture image.

Need a particular material quickly? This site has hundreds of ready to use materials, there's even a few by me there. It's also a great resource for learning how to create Blender materials as you can examine exactly how each was done in the blend file.