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Free high quality HDR panorama environment maps, from HDRI-Hub.com, that are licensed as Creative Commons Attribution 3.0 Unported are hard to ignore.
These are incredibly detailed and a must for anyone who is serious about photorealistic rendering. Creative Commons license means you a free to use these for your commercial work.

It is most exterior panoramas but nevertheless those exterior shots are sublime. From shaded forest scenes to stunning harbours there is something for everyone.
HDRI-Hub also have an extensive collection of backplates and textures that are also of a very high standard.
To grab your set head on over to the free sample section at www.hdri-hub.com.