Web site: http://www.insecure.org/nmap/
nmap is a powerful network-mapping and port-scanning tool. It is an indispensable weapon with a wide range of features. See nmap’s manual for a complete overview.
nmap supports many port-scanning techniques, ranging from the standard connect() function to SYN scanning:
TCP connect() scanning
TCP SYN (half-open) scanning
TCP FIN, Xmas, or NULL (stealth) scanning
TCP FTP proxy (bounce attack) scanning
SYN/FIN scanning using IP fragments
TCP ACK and window scanning
UDP raw ICMP port-unreachable scanning
TCP ping scanning
Direct (non-portmapper) RPC scanning
Remote OS ...