viewdns.info (2010-)

This is the most accessible DNS database online, as it does not require login or payment.

They have reasonable data. It's not fully complete as Ciro Santilli saw on CIA 2010 covert communication websites, but it is very valuable.

Tested as of 2025, they seem to have removed the pre-IP checks on web interface, and just instead use Cloudfare to check that you are human from time to time, which allows for a lot manual searching to be done! Awesome!

Previously, tou could only get about 250 queries on the web interface, then 250 queries per free account via API. They check your IP when you signup, and you can't sign in twice from the same IP. They also state that Tor addresses are blacklisted. They also normalize dots in gmail addresses, so you need more diverse email accounts. But they haven't covered the .gmail vs .googlemail trick.

Their data is also quite disjoint from the data of the 2013 DNS Census. There is some overlap, but clearly their methodology is very different. Some times they slot into one another almost perfectly.