Zbyg on DeviantArt has created three HDRi-packages that are free to use at least for personal work:
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
You might be allowed to use them for other types of work as well but I can’t remember how it was.
