There are still some lists of free DNS servers out there.
One of the oldest (and I think best) Free and Public DNS Servers. It is my go-to list.
Some group keeps a list of public DNS servers in the USA, with pages for lists of servers in other countries. You can download them in a csv file. There are also lists for a lot of other countries.
Another top ten list (with reasons you might want to use some of them) is at TechWhoop.
The Arch Linux Wiki has a good list including a short analysis of different servers (archive here). They point to a page on Wikipedia.
You could also go with OpenNIC. Here is their list of public Tier 2 servers. Their anycast servers do not work for me; perhaps my broadband provider is blocking them.
If you have a hard time remembering all of this, you can fall back on 1.1.1.1 (Cloudflare), 8.8.8.8 (Google) or 9.9.9.9 (Quad9). I tried X.X.X.X for 2 through 7, and got nothing.
I was using the DNS servers from Uncensored DNS, but they seem to be offline as of 2017-09-30. As of 2023-07-23, they are not responding to dig and host requests. Perhaps my broadband provider is censoring Uncensored DNS.
On Linux, you can test a DNS server with the host command: host $DOMAIN_TO_TEST $DNS_SERVER_IP_ADDRESS:
host google.com 8.8.8.8
or you can use the dig command: dig @$DNS_IP $DOMAIN_TO_TEST
dig @91.239.100.100 google.com
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