DNS Twister: https://dnstwister.report/
DNS Twist: https://github.com/elceef/dnstwist
Cat phish: https://github.com/ring0lab/catphish
Discover digital footprint - Shodan https://www.shodan.io/
Urlscan.io https://urlscan.io/search/#*
Redirect detective: https://redirectdetective.com/
Tool to crawl host names via SSL certs, and chain of trust between domains and the certs used between them https://github.com/lanrat/certgraph
Visual inspection of websites https://github.com/michenriksen/aquatone
Find subdomains - Spyse https://spyse.com/tools/subdomain-finder
The harvester https://github.com/laramies/theHarvester