The Network Mapper

 Nmap is a utility for network exploration or security auditing. It
 supports ping scanning (determine which hosts are up), many port
 scanning techniques, version detection (determine service protocols
 and application versions listening behind ports), and TCP/IP
 fingerprinting (remote host OS or device identification). Nmap also
 offers flexible target and port specification, decoy/stealth scanning,
 sunRPC scanning, and more. Most Unix and Windows platforms are
 supported in both GUI and commandline modes. Several popular handheld
 devices are also supported, including the Sharp Zaurus and the iPAQ.