Free HDRI Resources

High-dynamic-range imaging (HDRI or HDR) is a set of methods used in imaging and photography to capture a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging methods or photographic methods.
— Source: Wikipedia:

Here is a list of free HDRI Resources from around the internet.  Some of them include background plates to use as well.  I've had these bookmarked in my browser for easy access.  If you do use them, be sure to double check the license agreement whether you can use them for commercial use or not.