Episcura offers HDRI images - while it is a paid service, a free account allows you to download up to 15MB/day.
Chris Sikkenga writes:
episcura.com was born out of necessity. Textures available at stock sites are not affordable. HDR images are very expensive. A few CG artists decided to rectify the situation and created this new resource.
Our new home for seamless textures also offers HDRI and snapshots. The snapshots can be used to pitch ideas to clients, as references for models, or whatever you like. The textures and images are royalty-free.
Speaking of free, you can download 15MB daily for free. Monthly subscriptions are available which give users more download bandwidth. The site just launched and future plans include a free page that doesn't count against your bandwidth, 16 bit plates and a contest with a cash prize.
Please check out episcura.com.