Coding and Scripts

CSV Converters (convertcsv.com)
DHTML & Javascripts (dynamicdrive.com)
Markup Validation Service (validator.w3.org)
Minifier (codebeautify.org)
SEO Tools (technicalseo.com/seo-tools)
Structured Data Generator (technicalseo.com/ ... generator)
Structured Data Test (google.com/structured-data/testing-tool)

Content

Broken Link Checker (brokenlinkcheck.com)
Cookie Checker (cookieserve.com)
Dead Link Checker (deadlinkchecker.com)
Duplicate Content Checker (copyscape.com)
Duplicate Content Checker, Brocken Links, Etc. (Siteliner.com)
Real-time Visitor Statistics Widgets (revolvermaps.com)
Schema Markup Generator (technicalseo.com
Search Terms (answerthepublic.com)
Social Media Management (hootsuite.com)
Surveys (feedier.com)

Convert

Timestamp/Date (timestamp.online)

Email

DMARC/SPF/DKIM/BIMI Checker (easydmarc.com)
Email Verifier (email-checker.net)
Email Verifier (emailhippo.com)

Events

Ticketing Management (eventbrite.com)

Graphics

Favicon & App Icon Generator (favicon-generator.org)
Favicon & App Icon Generator (favicon.io)
Graphics Tool (autodraw.com)
Graphics Tool (canva.com)
Icon Database (IconArchive.com)
Icon Database (IconFinder.com)
Logo Maker (logomaker.com)
Logo Maker (looka.com)
Photo Editing and Effects (pho.to)

Images

Free Images (gratisography.com)
Free Images (1millionfreepictures.com)
Free Images (freepik.com)
Free Images (pixabay.com)
Free Images (splitshire.com)
Free Images (unsplash.com)
Free Vintage Images (nos.twnsnd.co)
Paid Images (123rf.com)
Paid Images (stock.adobe.com)

Performance

Browser Support Tables (caniuse.com)
DNS Checker (dnschecker.org)
DNS Test (leafdns.com)
Rendering Check (responsivetesttool.com)
SEO Analyzer (rankmath.com)
SEO Analyzer (seoptimer.com)
SEO Tools (seotoolstation.com)
Speed Test for Connection (speedtest.net)
Speed Test for Website (gtmetrix.com)
Speed Test for Website (tools.pingdom.com)
Website Grader (websitegrader.com)
Website Tester (nibbler.silktide.com)

QR Code

QR Code Generator (qrcode.kaywa.com)
QR Code Reader (zxing.org)

Security

Blocklist Removal Center (spamhaus.org)
DNSSEC Analyzer (verisignlabs.com)
Safe Browsing site status (google.com/safe-browsing/search)
SSL Test (ssllabs.com/ssltest)
SSL Tools (sslshopper.com/ssl-certificate-tools.html)
Website Safety Check (safeweb.norton.com)
Website Security Check (sitecheck.sucuri.net)

Specialty Search

Historical Search (archive.org)
Partial Domain Name Search (namedroppers.com)
Reverse Image Search (tineye.com)