|| P0f is a versatile passive OS fingerprinting tool. P0f can identify the system on machines that connect to your box, machines you connect to, and even machines that merely go thru or near your box. All this even if the device is behind a fascist packet firewall.
P0f will also detect what the remote system is hooked up to (be it Ethernet, DSL, OC3, or avian carriers), how far it is located, whats its uptime. The latest beta can also detect masquerade or illegal network hook-ups (useful for ISPs and corporate networks). P0f can detect certain types of packet filters and NAT setups, and sometimes can determine the name of the other guys ISP. Not a big deal? Its still passive. It does not generate any network traffic. No name lookups, no traffic to the victim, no ARIN queries, no traceroute.