DNS Dumpster
https://dnsdumpster.com/
Visual tool for looking up all DNS records associate with a domain. Provides a nice graph as well
nmap
Scans a host or set of for open ports
-- todo: add common and useful flags
Spider Foot
https://github.com/smicallef/spiderfoot
-- todo: add some usages here
Burp Suite
https://portswigger.net/burp/pro
Scan and fuzz target URLs to find and exploit vulnerabilities
Nessus
https://www.tenable.com/products/nessus/nessus-essentials
Scan hosts for vulnerabilities
Markdown Guide
https://www.markdownguide.org/basic-syntax/