Do YOU love HDRI's? Well, I do.

 Searching the web, I collected all the sites where you can download free HDRI's legally.
I hope that they will be of use for you!

• http://wiki.mcneel.com/people/brianjames#high_dynamic_range_environments
• http://gl.ict.usc.edu/Data/HighResProbes/

(^ they are not always online, so try it at a later time when you get to see no site)

• http://www.hdrlabs.com/sibl/archive.html

(^ they update their site regularly)

• http://www.evermotion.org/download
• http://dativ.at/lightprobes/
• http://www.turbosquid.com/FullPreview/Index.cfm/ID/256272
• http://www.digitecture.org/index.php/images/
• http://www.zwischendrin.com/de/browse/hdri/3

(^ the one's with "kostenlos")

• http://www.weiss-ag.info/gallerie/hdr-einzelbilder-shop/
• http://zbyg.deviantart.com/art/HDRi-Pack-1-97402522
• http://zbyg.deviantart.com/art/HDRi-Pack-2-103458406
• http://zbyg.deviantart.com/art/HDRi-Pack-3-112847728
• http://www.openfootage.net/?cat=15

(^ they have textures as well)

• http://www.hdrmill.com/Freebies.htm
• http://www.cg-cars.com/showthread.php/6146-2-full-res-HDR-amp-backplates-for-download-if-you-give-me-feedback!

• http://crazy8studio.com/hdri.html
• http://www.alecmoody.com/stuff/hdri_release/

(^ the 2 rar's)

• https://www.maground.com/download/MAGROUND_CGI_Set.zip
• http://www.hdri-locations.com/freesample.php
• http://www.hdri4free.com/
• http://www.mainworksshop.de/hdr360info.htmlMy
• http://www.cg-cars.com/showthread.php/6162-Free-HDRI-3-backplates

Personal tutorial about creating your own cheap HDRI (spherical 360 pano, for lighting purposes) if you happen to have an android phone and photoshop:
http://solidcookup.blogspot.com/2011/01/making-you-own-spherical-hdris-cheap.html

If you have found some other free HDRI's please comment, and I'll add them to the list.