#1Subdomain discovery, DNS brute-force, certificate transparency, and asset enumeration tools.
Kitploit recommended

Domains belonging to the most reputed public bug bounty programs. [NOT FOR NON-MONETARY OR PRIVATE PROGRAMS]

Nodesub is a command-line tool for finding subdomains in bug bounty programs

A Cloudflare resolver that works

This tool can be used to enumerate the subdomains associated with a company by aggregating the results of multiple OSINT (Open Source Intelligence)…

Dumain Bruteforcer - a fast and flexible domain bruteforcer

API-first subdomain discovery service using Certificate Transparency logs for fast, passive enumeration of subdomains via a REST API with JSON or…

OSINT tool abusing SecurityTrails domain suggestion API to find potentially related domains by keyword and brute force.