os fingerprint

#

nmap(client side)

http://www.insecure.org/nmap/

http://www.winpcap.org/

p0f (server side)

http://lcamtuf.coredump.cx/p0f.shtml

#

www.xuite.net

Running: Cisco IOS 12.X, WatchGuard embedded
OS details: Cisco 837 router running IOS 12.3(8)T, WatchGuard Firebox X700

tw.yahoo.com

Running: FreeBSD 5.X
OS details: FreeBSD 5.2.1 (SPARC), FreeBSD 5.4-RELEASE

ptt.cc

Running: Linux 2.4.X|2.6.X
OS details: Linux 2.4.18 - 2.4.27, Linux 2.4.21 (Suse, X86), Linux 2.4.22, Linux
 2.4.6 - 2.4.26 or 2.6.9, Linux 2.6.10, Linux 2.6.5 - 2.6.11, Linux 2.6.7, Linux
 2.6.8 (Debian)