p0f

P0f v2 is a versatile passive OS fingerprinting tool. P0f can identify the
operating system on:

- machines that connect to your box (SYN mode),
- machines you connect to (SYN+ACK mode),
- machine you cannot connect to (RST+ mode),
- machines whose communications you can observe.

Basically it can fingerprint anything, just by listening, it doesn’t make ANY
active connections to the target machine.

Get p0f Here - http://lcamtuf.coredump.cx/p0f/p0f.shtml