Nmap is the most widely used active reconnaissance tool in penetration testing.
nmap -sn 192.168.1.0/24 # Host discovery
nmap -sS -sV -O -A 192.168.1.10 # Full aggressive scan
nmap -p- 192.168.1.10 # All ports
nmap --script vuln 192.168.1.10 # Vulnerability scripts
nmap -oA results 192.168.1.10 # Export all formats