XPROBE: Active OS fingerprinting tool

"Remote OS identification using ICMP packets Xprobe allows you to determine what operating system is running on a remote host. It sends several packets to a host and analyses the returned ICMP packets. The tool automates a logic of OS fingerprinting methods called "X". Xprobe's functionality is comparable to the OS fingerprinting feature in nmap, but has several advantages over it: - Faster: A maximum of 4 packets are sent to determine the remote OS. - Can detect whether the host is up, so pinging is no longer necessary. - Stealthier: Does not send any malformed datagrams. - Can distinguish between many variants of Microsoft operating systems." read more...

Website: http://xprobe.sourceforge.net