Nmap is the most widely used port scanner and network discovery tool. Capabilities: host discovery, port scanning, service version detection, OS fingerprinting, script scanning (NSE).
Common Nmap scan types: -sS (SYN/stealth), -sV (version detection), -O (OS detection), -A (aggressive: version+OS+scripts). -p- scans all 65535 ports. Always get authorization before scanning.