what Linux Version is used on a Webserver

Is this feasible? Can I find out which distro is used to host specific webpages? Whois doesn't have info for this.

>>2
type in a url which doesn't exist on the server and look at the bottom of the page. If it's not properly configured you can get the linux distribution , apache version etc...
otherwise use nmap for fingerprinting the OS

Read SICP.

Check the Server MIME type of any page returned by the server, if the admin decided to disclose it, like Server: Apache/1.2.3 (Stallmanix) PHP/9

as >>3 said, nmap

is the shit. you can get lots of info with that. it's really worth to learn.