Coding and Scripts

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

Content

Dead Link Checker (deadlinkchecker.com)
Duplicate Content Checker (copyscape.com)
Real-time Visitor Statistics Widgets (revolvermaps.com)

Domains

Partial Domain Name Search (namedroppers.com)

Email

Email Verifier (verify-email.org)
Sender Policy Framework (openspf.org)

Events

Ticketing Management (eventbrite.com)

Graphics

Graphics Tool (autodraw.com)
Logo Maker (logomaker.com)

Images

Free Images (1millionfreepictures.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)
Rendering Check (quirktools.com/screenfly)
Speed Check (tools.pingdom.com)
Website Grader (websitegrader.com)

QR Code

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

Security

Blocklist Removal Center (spamhaus.org)
Safe Browsing site status (google.com/safe-browsing/search)
SSL Test (ssllabs.com/ssltest)
Website Safety Check (safeweb.norton.com)
Website Security Check (sitecheck.sucuri.net)