Adblock tests
https://adblock-tester.com (Simple)
https://adblock.turtlecute.org (More thorough)
https://blockads.fivefilters.org (Simple)
Browser/Ad tracking tests
https://coveryourtracks.eff.org
Bufferbloat rating tests
https://www.waveform.com/tools/bufferbloat
Connection benchmark tests
https://www.fast.com
DNS (generic) tests
https://www.perfect-privacy.com/en/tests/dns-leaktest
https://one.one.one.one/help
DNSSEC tests
https://rootcanary.org/test.html
DNS (speed) tests
https://dnsspeedtest.online/
External Port tests
https://canyouseeme.org/
Multitest sites