TCP SYN scan
nmap -sS
TCP Connect() scan
nmap -sT
TCP ACK scan
nmap -sA
TCP NULL scan
nmap -sN
TCP FIN scan
nmap -sF
TCP Xmas scan
nmap -sX
UDP scan
nmap -sU
Service/version scan
nmap -sV
OS detection scan
nmap -O
Aggressive scan
nmap -A
Probe only (host discovery, not port scan)
nmap -sn