You can look for OS fingerprinting functionality built into NMap.

However, if you're looking for something like "What is Google running?" you won't get far since you won't know what's behind their load balancers, or firewalls will block it, and fingerprinting can only be so accurate so you can get false reports back and you may not get anywhere when the connection is somehow NAT'ed.

How can I find out which server OS a particular site is running?
I've been playing for a while now, I'm apparently looking for a girl or something, but unless I missed it, there was absolutely no introduction to who I am, or what I am doing, what I'm looking for, where I am, what any of this is.

Have I missed something (perhaps cos I didn't play the earlier bioshock games?)  or am I supposed to have no idea what's going on?
