I thought we were due for a nice group of links to free texture and reference images. Of course there are too many to list here, but here are a few that I've visited on occasion. Some of these are standards for free images, but not everyone knows they're out there.
- Noctua Graphics
Great straight-on 512x512 textures.
- Tor Oera's Homepage
There are several sites out there with Earth, Moon, and Mars maps but Tor also throws in a beginner tutorial on how to use the Earth maps.
- Planet Pixel Emporium
ALL THE PLANETS IN TEXTURE FORM
- CG India
Free textures for personal use only. The texture maps are downloadable through the website in .RAR format at are at a medium resolution.
- Art Morgue
A reference material site with a wide array of reference types with over 6,000 images. You can view the images, but if you want the higher res you'll need to join (which is free).
- Morgue File
Here is what they say: "A place to keep post production materials for use of reference, an inactive job file. This morgue file contains free high resolution digital stock photography for either corporate or public use."
- Thomas Surland's Page
Lots of blueprints for cars, planes, and other vehicles.
Last, but certainly not least. Got3D is very well known for textures and reference images, but if you haven't visited, take a look. They have a for money and a for free section. They for purchase section has a lot , but the free one has some nice ones too.