Web Links

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.

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.