Check an email address to validate it for deliverability, without sending an email that gets bounced/rejected: https://gsuite.tools/verify-email
Check to see if your website's DNS settings (e.g. CNAME, MX, TXT, A, etc.) are properly configured and accessible accross the internet: https://www.whatsmydns.net
Custom box templates in SVG for [laser] cutting projects: https://cutgen.io
Wide variety of single-function calculators, from IRA growth to BMI: https://www.calculator.net/sitemap.html
Variety of graphing, scientific, and even 3D calculators; simple user interface: https://www.geogebra.org/classic
Adobe Photoshop-like editor: https://www.photopea.com
Remove voice from MP3s, download separate MP3 files: https://vocalremover.org
Deep-fake text-to-speech audio generator: https://fakeyou.com
Send your friends here to help make them more efficient communicators: https://nohello.net/
Make QR codes that also depict an Ai description prompt: https://huggingface.co/spaces/huggingface-projects/QR-code-AI-art-generator
Read the most important/interesting sections of a terms of service (that's way too long to read): https://tosdr.org/en/frontpage
Find a phone number more likely to reach a human: https://gethuman.com
Make posters without large paper, by formatting images to print across multiple pages of typing paper: https://rasterbator.net
Convert a 3D (CAD) object into a 2D shape, print-out-able, and foldable-back-into-the-3D-version: https://pepakura.tamasoft.co.jp/pepakura_designer/
Find learning resources for lots of topics (but your topic has to be auto-completable in their database): https://learn-anything.xyz