SEO tools

SEO Audit

Screaming Frog
Integrity (for Mac users) and Xenu Sleuth (for PC users)
SEO Browser
Wayback Machine
Moz
BuzzSumo
Deep-crawl
Copyscape
Google Tag Manager
Google Tag Manager Chrome Extension
Annie Cushing’s Campaign Tagging Guide
Google Analytics (if given access)
Google Search Console (if given access)
Bing Webmaster Tools (if given access)
You Get Signal
Pingdom
PageSpeed Tool

Technical