From this site, you can either perform free historic reverse DNS lookups, or purchase our compressed and searchable reverse DNS data.
Free Reverse DNS Lookups
At this site, you can perform limited reverse DNS lookups at no cost.
You may not use automated programs (such as wget) to download data from the site automatically. In the past on multiple occasions on networking sites I have had, people wrote programs that would download data automatically, and they distributed the programs to others. Do not think about doing that (unless you have permission); once it is out there, the hits to the site never stop. If it happens, I may have to resort to setting up the site to return bogus data.
Paying for Data
I also offer data and software for purchase. In this case, you get reverse DNS data in a highly compressed form and software that can quickly and easily search the data.
Some of this data is available elsewhere at no cost (e.g. at Project Sonar). If you are comfortable handling and processing large data (e.g. 50GB+ files), go for it. I provide 'value added' services -- you can just go ahead and access the data immediately (typically taking under 5GB total storage space), and may have some extras (such as delegation data).