Coding and Scripts

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

Content

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

Domains

namedroppers.com (Partial Domain Name Search)

Email

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

Events

eventbrite.com (Ticketing Management)

Graphics

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

Images

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

Performance

quirktools.com/screenfly (Rendering Check)
tools.pingdom.com (Website Speed Check)
websitegrader.com (Website Grader)

QR Code

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

Security

ssllabs.com/ssltest (SSL Test)
sitecheck.sucuri.net (Website Security Check)