(It's always dns.)
https://isitdns.com
https://shouldiblamecaching.com
https://bofh.d00t.org
nc bofh.jeffballard.us 666 # I am surprised this is still working given its age; the CGI version at https://pages.cs.wisc.edu/~ballard/bofh/ doesn't work anymore (but the page is still up)
(It's always dns.)