Relies on Google Upstream Services, Plugins or Infrastructure:

Chime - SMTP
Capital District Transit Authority - SDK, Maps, License Check
Transit App - SDK, Maps
Rand McNally - Maps
Uber - SDK, Maps, License Check
Lyft - SDK, Maps, License Check
Trend Micro - SDK, License Check
Chargebee - Cloud
Monarch Investments - API, Payments
United Group of Companies - API, Payments
Apartments.com - reCaptcha (support form)
Avail.co - SMTP
Buckeye Broadband - reCaptcha (quote form)
Domino's - AJAX
Amtrak - AJAX
Pay With Privacy - SMTP
The Literacy Lab - SMTP
DigitalOcean - SMTP
Mint Mobile - SMTP
Vultr - SMTP
Equifax - SMTP
Khan Academy - SMTP
Little Ceasars - AJAX
Mongabay - Mailchimp
BioElectricShield - Mailchimp
DNSimple - SMTP
Yelp - SMTP
Freewallet - SMTP
LocalBitcoins - SMTP
Coinbase - SMTP, reCaptcha
ServerWallet - SMTP
IHeartRadio - SMTP
Github - SMTP
Daily Stormer - Domains, DNS
LinuxQuestions.org - Domains, DNS
ZDNet - Domains, DNS
Health Ranger Store - reCaptcha
Mailchimp - SMTP
Gigablast - SMTP
kansas-city.social - Domains
social.hendrixgames.com - Domains
mastofeed.com - DNS
BubbleTea Dev - Domains***
TechSpot - Domains, DNS
The Joyo - Domains, DNS
KCL Academy - DNS
Strelizia Network - Domains, DNS
Floss Social - Domains, DNS
Viridian Patriots - Domains, DNS
Binary Dad Social - Domains, DNS
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Discord - Physical Infrastructure, reCaptcha
https://web.archive.org/web/20200110151714/https://www.reddit.com/r/discordapp/comments/e7huer/discord_outage_7_december_2019/
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*** - this instance is using Cloudflare DNS, so it is visible to the Krowverse.