It's almost 24GB of data - you don't have to download everything at once. Your files will be always available at My Downloads after logging in.
Since version 3 of their rendering suite, in resources browser, you can find folder HDRMAPS with 4 hdri maps. With Maxwell Render, you can discover how good they are!
You can find few hdri from HDRMAPS in this amazing baking textures pocket knife. HDRi Map which was taken at Tenerife is probably the most used by Toolbag's users.
Previously by Allegorithmic now Adobe, Substance Suite is an industry standard for texture painting and materials creation. Most of the hdri you can find in this package come from hdrmaps.com
Tao is the most recent plugin, part of Trapcode Suite developed by talented Swedish programmer Peder Norrby. It comes with 7 hdri from HDRMAPS. They have to be downloaded directly from RedGiant web page.