Just a little site that was brought to my attention. No doubt this will be invaluable to anyone in the CG world, or even if you’re just really into photography.
Check out the sIBL archive for tons of free HDRI panoramas and images, all listed under the creative commons license. The website also features open source software you can use to edit the images and setup the lighting in your favorite 3d application (3dsmax, maya, xsi…etc.).
HDRI images all come in the .ibl format, read about the format specs here.
You can use the images however you like, so long as you say where you got them from.